esprima.js

/*
  Copyright (c) jQuery Foundation, Inc. and Contributors, All Rights Reserved.

  Redistribution and use in source and binary forms, with or without
  modification, are permitted provided that the following conditions are met:

    * Redistributions of source code must retain the above copyright
      notice, this list of conditions and the following disclaimer.
    * Redistributions in binary form must reproduce the above copyright
      notice, this list of conditions and the following disclaimer in the
      documentation and/or other materials provided with the distribution.

  THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
  AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
  IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
  ARE DISCLAIMED. IN NO EVENT SHALL <COPYRIGHT HOLDER> BE LIABLE FOR ANY
  DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
  (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
  LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND
  ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
  (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
  THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
*/

(function (root, factory) {
    'use strict';

    // Universal Module Definition (UMD) to support AMD, CommonJS/Node.js,
    // Rhino, and plain browser loading.

    /* istanbul ignore next */
    if (typeof define === 'function' && define.amd) {
        define(['exports'], factory);
    } else if (typeof exports !== 'undefined') {
        factory(exports);
    } else {
        factory((root.esprima = {}));
    }
}(this, function (exports) {
    'use strict';

    var Token,
        TokenName,
        FnExprTokens,
        Syntax,
        PlaceHolders,
        Messages,
        Regex,
        source,
        strict,
        sourceType,
        index,
        lineNumber,
        lineStart,
        hasLineTerminator,
        lastIndex,
        lastLineNumber,
        lastLineStart,
        startIndex,
        startLineNumber,
        startLineStart,
        scanning,
        length,
        lookahead,
        state,
        extra,
        isBindingElement,
        isAssignmentTarget,
        firstCoverInitializedNameError;

    Token = {
        BooleanLiteral: 1,
        EOF: 2,
        Identifier: 3,
        Keyword: 4,
        NullLiteral: 5,
        NumericLiteral: 6,
        Punctuator: 7,
        StringLiteral: 8,
        RegularExpression: 9,
        Template: 10
    };

    TokenName = {};
    TokenName[Token.BooleanLiteral] = 'Boolean';
    TokenName[Token.EOF] = '<end>';
    TokenName[Token.Identifier] = 'Identifier';
    TokenName[Token.Keyword] = 'Keyword';
    TokenName[Token.NullLiteral] = 'Null';
    TokenName[Token.NumericLiteral] = 'Numeric';
    TokenName[Token.Punctuator] = 'Punctuator';
    TokenName[Token.StringLiteral] = 'String';
    TokenName[Token.RegularExpression] = 'RegularExpression';
    TokenName[Token.Template] = 'Template';

    // A function following one of those tokens is an expression.
    FnExprTokens = ['(', '{', '[', 'in', 'typeof', 'instanceof', 'new',
                    'return', 'case', 'delete', 'throw', 'void',
                    // assignment operators
                    '=', '+=', '-=', '*=', '/=', '%=', '<<=', '>>=', '>>>=',
                    '&=', '|=', '^=', ',',
                    // binary/unary operators
                    '+', '-', '*', '/', '%', '++', '--', '<<', '>>', '>>>', '&',
                    '|', '^', '!', '~', '&&', '||', '?', ':', '===', '==', '>=',
                    '<=', '<', '>', '!=', '!=='];

    Syntax = {
        AssignmentExpression: 'AssignmentExpression',
        AssignmentPattern: 'AssignmentPattern',
        ArrayExpression: 'ArrayExpression',
        ArrayPattern: 'ArrayPattern',
        ArrowFunctionExpression: 'ArrowFunctionExpression',
        BlockStatement: 'BlockStatement',
        BinaryExpression: 'BinaryExpression',
        BreakStatement: 'BreakStatement',
        CallExpression: 'CallExpression',
        CatchClause: 'CatchClause',
        ClassBody: 'ClassBody',
        ClassDeclaration: 'ClassDeclaration',
        ClassExpression: 'ClassExpression',
        ConditionalExpression: 'ConditionalExpression',
        ContinueStatement: 'ContinueStatement',
        DoWhileStatement: 'DoWhileStatement',
        DebuggerStatement: 'DebuggerStatement',
        EmptyStatement: 'EmptyStatement',
        ExportAllDeclaration: 'ExportAllDeclaration',
        ExportDefaultDeclaration: 'ExportDefaultDeclaration',
        ExportNamedDeclaration: 'ExportNamedDeclaration',
        ExportSpecifier: 'ExportSpecifier',
        ExpressionStatement: 'ExpressionStatement',
        ForStatement: 'ForStatement',
        ForOfStatement: 'ForOfStatement',
        ForInStatement: 'ForInStatement',
        FunctionDeclaration: 'FunctionDeclaration',
        FunctionExpression: 'FunctionExpression',
        Identifier: 'Identifier',
        IfStatement: 'IfStatement',
        ImportDeclaration: 'ImportDeclaration',
        ImportDefaultSpecifier: 'ImportDefaultSpecifier',
        ImportNamespaceSpecifier: 'ImportNamespaceSpecifier',
        ImportSpecifier: 'ImportSpecifier',
        Literal: 'Literal',
        LabeledStatement: 'LabeledStatement',
        LogicalExpression: 'LogicalExpression',
        MemberExpression: 'MemberExpression',
        MethodDefinition: 'MethodDefinition',
        NewExpression: 'NewExpression',
        ObjectExpression: 'ObjectExpression',
        ObjectPattern: 'ObjectPattern',
        Program: 'Program',
        Property: 'Property',
        RestElement: 'RestElement',
        ReturnStatement: 'ReturnStatement',
        SequenceExpression: 'SequenceExpression',
        SpreadElement: 'SpreadElement',
        Super: 'Super',
        SwitchCase: 'SwitchCase',
        SwitchStatement: 'SwitchStatement',
        TaggedTemplateExpression: 'TaggedTemplateExpression',
        TemplateElement: 'TemplateElement',
        TemplateLiteral: 'TemplateLiteral',
        ThisExpression: 'ThisExpression',
        ThrowStatement: 'ThrowStatement',
        TryStatement: 'TryStatement',
        UnaryExpression: 'UnaryExpression',
        UpdateExpression: 'UpdateExpression',
        VariableDeclaration: 'VariableDeclaration',
        VariableDeclarator: 'VariableDeclarator',
        WhileStatement: 'WhileStatement',
        WithStatement: 'WithStatement',
        YieldExpression: 'YieldExpression'
    };

    PlaceHolders = {
        ArrowParameterPlaceHolder: 'ArrowParameterPlaceHolder'
    };

    // Error messages should be identical to V8.
    Messages = {
        UnexpectedToken: 'Unexpected token %0',
        UnexpectedNumber: 'Unexpected number',
        UnexpectedString: 'Unexpected string',
        UnexpectedIdentifier: 'Unexpected identifier',
        UnexpectedReserved: 'Unexpected reserved word',
        UnexpectedTemplate: 'Unexpected quasi %0',
        UnexpectedEOS: 'Unexpected end of input',
        NewlineAfterThrow: 'Illegal newline after throw',
        InvalidRegExp: 'Invalid regular expression',
        UnterminatedRegExp: 'Invalid regular expression: missing /',
        InvalidLHSInAssignment: 'Invalid left-hand side in assignment',
        InvalidLHSInForIn: 'Invalid left-hand side in for-in',
        InvalidLHSInForLoop: 'Invalid left-hand side in for-loop',
        MultipleDefaultsInSwitch: 'More than one default clause in switch statement',
        NoCatchOrFinally: 'Missing catch or finally after try',
        UnknownLabel: 'Undefined label \'%0\'',
        Redeclaration: '%0 \'%1\' has already been declared',
        IllegalContinue: 'Illegal continue statement',
        IllegalBreak: 'Illegal break statement',
        IllegalReturn: 'Illegal return statement',
        IllegalYield: 'Unexpected token yield',
        StrictModeWith: 'Strict mode code may not include a with statement',
        StrictCatchVariable: 'Catch variable may not be eval or arguments in strict mode',
        StrictVarName: 'Variable name may not be eval or arguments in strict mode',
        StrictParamName: 'Parameter name eval or arguments is not allowed in strict mode',
        StrictParamDupe: 'Strict mode function may not have duplicate parameter names',
        StrictFunctionName: 'Function name may not be eval or arguments in strict mode',
        StrictOctalLiteral: 'Octal literals are not allowed in strict mode.',
        StrictDelete: 'Delete of an unqualified identifier in strict mode.',
        StrictLHSAssignment: 'Assignment to eval or arguments is not allowed in strict mode',
        StrictLHSPostfix: 'Postfix increment/decrement may not have eval or arguments operand in strict mode',
        StrictLHSPrefix: 'Prefix increment/decrement may not have eval or arguments operand in strict mode',
        StrictReservedWord: 'Use of future reserved word in strict mode',
        TemplateOctalLiteral: 'Octal literals are not allowed in template strings.',
        ParameterAfterRestParameter: 'Rest parameter must be last formal parameter',
        DefaultRestParameter: 'Unexpected token =',
        ObjectPatternAsRestParameter: 'Unexpected token {',
        DuplicateProtoProperty: 'Duplicate __proto__ fields are not allowed in object literals',
        ConstructorSpecialMethod: 'Class constructor may not be an accessor',
        DuplicateConstructor: 'A class may only have one constructor',
        StaticPrototype: 'Classes may not have static property named prototype',
        MissingFromClause: 'Unexpected token',
        NoAsAfterImportNamespace: 'Unexpected token',
        InvalidModuleSpecifier: 'Unexpected token',
        IllegalImportDeclaration: 'Unexpected token',
        IllegalExportDeclaration: 'Unexpected token'
    };

    // See also tools/generate-unicode-regex.py.
    Regex = {
        NonAsciiIdentifierStart: new RegExp('[\xAA\xB5\xBA\xC0-\xD6\xD8-\xF6\xF8-\u02C1\u02C6-\u02D1\u02E0-\u02E4\u02EC\u02EE\u0370-\u0374\u0376\u0377\u037A-\u037D\u037F\u0386\u0388-\u038A\u038C\u038E-\u03A1\u03A3-\u03F5\u03F7-\u0481\u048A-\u052F\u0531-\u0556\u0559\u0561-\u0587\u05D0-\u05EA\u05F0-\u05F2\u0620-\u064A\u066E\u066F\u0671-\u06D3\u06D5\u06E5\u06E6\u06EE\u06EF\u06FA-\u06FC\u06FF\u0710\u0712-\u072F\u074D-\u07A5\u07B1\u07CA-\u07EA\u07F4\u07F5\u07FA\u0800-\u0815\u081A\u0824\u0828\u0840-\u0858\u08A0-\u08B2\u0904-\u0939\u093D\u0950\u0958-\u0961\u0971-\u0980\u0985-\u098C\u098F\u0990\u0993-\u09A8\u09AA-\u09B0\u09B2\u09B6-\u09B9\u09BD\u09CE\u09DC\u09DD\u09DF-\u09E1\u09F0\u09F1\u0A05-\u0A0A\u0A0F\u0A10\u0A13-\u0A28\u0A2A-\u0A30\u0A32\u0A33\u0A35\u0A36\u0A38\u0A39\u0A59-\u0A5C\u0A5E\u0A72-\u0A74\u0A85-\u0A8D\u0A8F-\u0A91\u0A93-\u0AA8\u0AAA-\u0AB0\u0AB2\u0AB3\u0AB5-\u0AB9\u0ABD\u0AD0\u0AE0\u0AE1\u0B05-\u0B0C\u0B0F\u0B10\u0B13-\u0B28\u0B2A-\u0B30\u0B32\u0B33\u0B35-\u0B39\u0B3D\u0B5C\u0B5D\u0B5F-\u0B61\u0B71\u0B83\u0B85-\u0B8A\u0B8E-\u0B90\u0B92-\u0B95\u0B99\u0B9A\u0B9C\u0B9E\u0B9F\u0BA3\u0BA4\u0BA8-\u0BAA\u0BAE-\u0BB9\u0BD0\u0C05-\u0C0C\u0C0E-\u0C10\u0C12-\u0C28\u0C2A-\u0C39\u0C3D\u0C58\u0C59\u0C60\u0C61\u0C85-\u0C8C\u0C8E-\u0C90\u0C92-\u0CA8\u0CAA-\u0CB3\u0CB5-\u0CB9\u0CBD\u0CDE\u0CE0\u0CE1\u0CF1\u0CF2\u0D05-\u0D0C\u0D0E-\u0D10\u0D12-\u0D3A\u0D3D\u0D4E\u0D60\u0D61\u0D7A-\u0D7F\u0D85-\u0D96\u0D9A-\u0DB1\u0DB3-\u0DBB\u0DBD\u0DC0-\u0DC6\u0E01-\u0E30\u0E32\u0E33\u0E40-\u0E46\u0E81\u0E82\u0E84\u0E87\u0E88\u0E8A\u0E8D\u0E94-\u0E97\u0E99-\u0E9F\u0EA1-\u0EA3\u0EA5\u0EA7\u0EAA\u0EAB\u0EAD-\u0EB0\u0EB2\u0EB3\u0EBD\u0EC0-\u0EC4\u0EC6\u0EDC-\u0EDF\u0F00\u0F40-\u0F47\u0F49-\u0F6C\u0F88-\u0F8C\u1000-\u102A\u103F\u1050-\u1055\u105A-\u105D\u1061\u1065\u1066\u106E-\u1070\u1075-\u1081\u108E\u10A0-\u10C5\u10C7\u10CD\u10D0-\u10FA\u10FC-\u1248\u124A-\u124D\u1250-\u1256\u1258\u125A-\u125D\u1260-\u1288\u128A-\u128D\u1290-\u12B0\u12B2-\u12B5\u12B8-\u12BE\u12C0\u12C2-\u12C5\u12C8-\u12D6\u12D8-\u1310\u1312-\u1315\u1318-\u135A\u1380-\u138F\u13A0-\u13F4\u1401-\u166C\u166F-\u167F\u1681-\u169A\u16A0-\u16EA\u16EE-\u16F8\u1700-\u170C\u170E-\u1711\u1720-\u1731\u1740-\u1751\u1760-\u176C\u176E-\u1770\u1780-\u17B3\u17D7\u17DC\u1820-\u1877\u1880-\u18A8\u18AA\u18B0-\u18F5\u1900-\u191E\u1950-\u196D\u1970-\u1974\u1980-\u19AB\u19C1-\u19C7\u1A00-\u1A16\u1A20-\u1A54\u1AA7\u1B05-\u1B33\u1B45-\u1B4B\u1B83-\u1BA0\u1BAE\u1BAF\u1BBA-\u1BE5\u1C00-\u1C23\u1C4D-\u1C4F\u1C5A-\u1C7D\u1CE9-\u1CEC\u1CEE-\u1CF1\u1CF5\u1CF6\u1D00-\u1DBF\u1E00-\u1F15\u1F18-\u1F1D\u1F20-\u1F45\u1F48-\u1F4D\u1F50-\u1F57\u1F59\u1F5B\u1F5D\u1F5F-\u1F7D\u1F80-\u1FB4\u1FB6-\u1FBC\u1FBE\u1FC2-\u1FC4\u1FC6-\u1FCC\u1FD0-\u1FD3\u1FD6-\u1FDB\u1FE0-\u1FEC\u1FF2-\u1FF4\u1FF6-\u1FFC\u2071\u207F\u2090-\u209C\u2102\u2107\u210A-\u2113\u2115\u2119-\u211D\u2124\u2126\u2128\u212A-\u212D\u212F-\u2139\u213C-\u213F\u2145-\u2149\u214E\u2160-\u2188\u2C00-\u2C2E\u2C30-\u2C5E\u2C60-\u2CE4\u2CEB-\u2CEE\u2CF2\u2CF3\u2D00-\u2D25\u2D27\u2D2D\u2D30-\u2D67\u2D6F\u2D80-\u2D96\u2DA0-\u2DA6\u2DA8-\u2DAE\u2DB0-\u2DB6\u2DB8-\u2DBE\u2DC0-\u2DC6\u2DC8-\u2DCE\u2DD0-\u2DD6\u2DD8-\u2DDE\u2E2F\u3005-\u3007\u3021-\u3029\u3031-\u3035\u3038-\u303C\u3041-\u3096\u309D-\u309F\u30A1-\u30FA\u30FC-\u30FF\u3105-\u312D\u3131-\u318E\u31A0-\u31BA\u31F0-\u31FF\u3400-\u4DB5\u4E00-\u9FCC\uA000-\uA48C\uA4D0-\uA4FD\uA500-\uA60C\uA610-\uA61F\uA62A\uA62B\uA640-\uA66E\uA67F-\uA69D\uA6A0-\uA6EF\uA717-\uA71F\uA722-\uA788\uA78B-\uA78E\uA790-\uA7AD\uA7B0\uA7B1\uA7F7-\uA801\uA803-\uA805\uA807-\uA80A\uA80C-\uA822\uA840-\uA873\uA882-\uA8B3\uA8F2-\uA8F7\uA8FB\uA90A-\uA925\uA930-\uA946\uA960-\uA97C\uA984-\uA9B2\uA9CF\uA9E0-\uA9E4\uA9E6-\uA9EF\uA9FA-\uA9FE\uAA00-\uAA28\uAA40-\uAA42\uAA44-\uAA4B\uAA60-\uAA76\uAA7A\uAA7E-\uAAAF\uAAB1\uAAB5\uAAB6\uAAB9-\uAABD\uAAC0\uAAC2\uAADB-\uAADD\uAAE0-\uAAEA\uAAF2-\uAAF4\uAB01-\uAB06\uAB09-\uAB0E\uAB11-\uAB16\uAB20-\uAB26\uAB28-\uAB2E\uAB30-\uAB5A\uAB5C-\uAB5F\uAB64\uAB65\uABC0-\uABE2\uAC00-\uD7A3\uD7B0-\uD7C6\uD7CB-\uD7FB\uF900-\uFA6D\uFA70-\uFAD9\uFB00-\uFB06\uFB13-\uFB17\uFB1D\uFB1F-\uFB28\uFB2A-\uFB36\uFB38-\uFB3C\uFB3E\uFB40\uFB41\uFB43\uFB44\uFB46-\uFBB1\uFBD3-\uFD3D\uFD50-\uFD8F\uFD92-\uFDC7\uFDF0-\uFDFB\uFE70-\uFE74\uFE76-\uFEFC\uFF21-\uFF3A\uFF41-\uFF5A\uFF66-\uFFBE\uFFC2-\uFFC7\uFFCA-\uFFCF\uFFD2-\uFFD7\uFFDA-\uFFDC]'),
        NonAsciiIdentifierPart: new 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    };

    // Ensure the condition is true, otherwise throw an error.
    // This is only to have a better contract semantic, i.e. another safety net
    // to catch a logic error. The condition shall be fulfilled in normal case.
    // Do NOT use this to enforce a certain condition on any user input.

    function assert(condition, message) {
        /* istanbul ignore if */
        if (!condition) {
            throw new Error('ASSERT: ' + message);
        }
    }

    function isDecimalDigit(ch) {
        return (ch >= 0x30 && ch <= 0x39);   // 0..9
    }

    function isHexDigit(ch) {
        return '0123456789abcdefABCDEF'.indexOf(ch) >= 0;
    }

    function isOctalDigit(ch) {
        return '01234567'.indexOf(ch) >= 0;
    }

    function octalToDecimal(ch) {
        // \0 is not octal escape sequence
        var octal = (ch !== '0'), code = '01234567'.indexOf(ch);

        if (index < length && isOctalDigit(source[index])) {
            octal = true;
            code = code * 8 + '01234567'.indexOf(source[index++]);

            // 3 digits are only allowed when string starts
            // with 0, 1, 2, 3
            if ('0123'.indexOf(ch) >= 0 &&
                    index < length &&
                    isOctalDigit(source[index])) {
                code = code * 8 + '01234567'.indexOf(source[index++]);
            }
        }

        return {
            code: code,
            octal: octal
        };
    }

    // 7.2 White Space

    function isWhiteSpace(ch) {
        return (ch === 0x20) || (ch === 0x09) || (ch === 0x0B) || (ch === 0x0C) || (ch === 0xA0) ||
            (ch >= 0x1680 && [0x1680, 0x180E, 0x2000, 0x2001, 0x2002, 0x2003, 0x2004, 0x2005, 0x2006, 0x2007, 0x2008, 0x2009, 0x200A, 0x202F, 0x205F, 0x3000, 0xFEFF].indexOf(ch) >= 0);
    }

    // 7.3 Line Terminators

    function isLineTerminator(ch) {
        return (ch === 0x0A) || (ch === 0x0D) || (ch === 0x2028) || (ch === 0x2029);
    }

    // 7.6 Identifier Names and Identifiers

    function isIdentifierStart(ch) {
        return (ch === 0x24) || (ch === 0x5F) ||  // $ (dollar) and _ (underscore)
            (ch >= 0x41 && ch <= 0x5A) ||         // A..Z
            (ch >= 0x61 && ch <= 0x7A) ||         // a..z
            (ch === 0x5C) ||                      // \ (backslash)
            ((ch >= 0x80) && Regex.NonAsciiIdentifierStart.test(String.fromCharCode(ch)));
    }

    function isIdentifierPart(ch) {
        return (ch === 0x24) || (ch === 0x5F) ||  // $ (dollar) and _ (underscore)
            (ch >= 0x41 && ch <= 0x5A) ||         // A..Z
            (ch >= 0x61 && ch <= 0x7A) ||         // a..z
            (ch >= 0x30 && ch <= 0x39) ||         // 0..9
            (ch === 0x5C) ||                      // \ (backslash)
            ((ch >= 0x80) && Regex.NonAsciiIdentifierPart.test(String.fromCharCode(ch)));
    }

    // 7.6.1.2 Future Reserved Words

    function isFutureReservedWord(id) {
        switch (id) {
        case 'enum':
        case 'export':
        case 'import':
        case 'super':
            return true;
        default:
            return false;
        }
    }

    // 11.6.2.2 Future Reserved Words

    function isStrictModeReservedWord(id) {
        switch (id) {
        case 'implements':
        case 'interface':
        case 'package':
        case 'private':
        case 'protected':
        case 'public':
        case 'static':
        case 'yield':
        case 'let':
            return true;
        default:
            return false;
        }
    }

    function isRestrictedWord(id) {
        return id === 'eval' || id === 'arguments';
    }

    // 7.6.1.1 Keywords

    function isKeyword(id) {

        // 'const' is specialized as Keyword in V8.
        // 'yield' and 'let' are for compatibility with SpiderMonkey and ES.next.
        // Some others are from future reserved words.

        switch (id.length) {
        case 2:
            return (id === 'if') || (id === 'in') || (id === 'do');
        case 3:
            return (id === 'var') || (id === 'for') || (id === 'new') ||
                (id === 'try') || (id === 'let');
        case 4:
            return (id === 'this') || (id === 'else') || (id === 'case') ||
                (id === 'void') || (id === 'with') || (id === 'enum');
        case 5:
            return (id === 'while') || (id === 'break') || (id === 'catch') ||
                (id === 'throw') || (id === 'const') || (id === 'yield') ||
                (id === 'class') || (id === 'super');
        case 6:
            return (id === 'return') || (id === 'typeof') || (id === 'delete') ||
                (id === 'switch') || (id === 'export') || (id === 'import');
        case 7:
            return (id === 'default') || (id === 'finally') || (id === 'extends');
        case 8:
            return (id === 'function') || (id === 'continue') || (id === 'debugger');
        case 10:
            return (id === 'instanceof');
        default:
            return false;
        }
    }

    // 7.4 Comments

    function addComment(type, value, start, end, loc) {
        var comment;

        assert(typeof start === 'number', 'Comment must have valid position');

        state.lastCommentStart = start;

        comment = {
            type: type,
            value: value
        };
        if (extra.range) {
            comment.range = [start, end];
        }
        if (extra.loc) {
            comment.loc = loc;
        }
        extra.comments.push(comment);
        if (extra.attachComment) {
            extra.leadingComments.push(comment);
            extra.trailingComments.push(comment);
        }
    }

    function skipSingleLineComment(offset) {
        var start, loc, ch, comment;

        start = index - offset;
        loc = {
            start: {
                line: lineNumber,
                column: index - lineStart - offset
            }
        };

        while (index < length) {
            ch = source.charCodeAt(index);
            ++index;
            if (isLineTerminator(ch)) {
                hasLineTerminator = true;
                if (extra.comments) {
                    comment = source.slice(start + offset, index - 1);
                    loc.end = {
                        line: lineNumber,
                        column: index - lineStart - 1
                    };
                    addComment('Line', comment, start, index - 1, loc);
                }
                if (ch === 13 && source.charCodeAt(index) === 10) {
                    ++index;
                }
                ++lineNumber;
                lineStart = index;
                return;
            }
        }

        if (extra.comments) {
            comment = source.slice(start + offset, index);
            loc.end = {
                line: lineNumber,
                column: index - lineStart
            };
            addComment('Line', comment, start, index, loc);
        }
    }

    function skipMultiLineComment() {
        var start, loc, ch, comment;

        if (extra.comments) {
            start = index - 2;
            loc = {
                start: {
                    line: lineNumber,
                    column: index - lineStart - 2
                }
            };
        }

        while (index < length) {
            ch = source.charCodeAt(index);
            if (isLineTerminator(ch)) {
                if (ch === 0x0D && source.charCodeAt(index + 1) === 0x0A) {
                    ++index;
                }
                hasLineTerminator = true;
                ++lineNumber;
                ++index;
                lineStart = index;
            } else if (ch === 0x2A) {
                // Block comment ends with '*/'.
                if (source.charCodeAt(index + 1) === 0x2F) {
                    ++index;
                    ++index;
                    if (extra.comments) {
                        comment = source.slice(start + 2, index - 2);
                        loc.end = {
                            line: lineNumber,
                            column: index - lineStart
                        };
                        addComment('Block', comment, start, index, loc);
                    }
                    return;
                }
                ++index;
            } else {
                ++index;
            }
        }

        // Ran off the end of the file - the whole thing is a comment
        if (extra.comments) {
            loc.end = {
                line: lineNumber,
                column: index - lineStart
            };
            comment = source.slice(start + 2, index);
            addComment('Block', comment, start, index, loc);
        }
        tolerateUnexpectedToken();
    }

    function skipComment() {
        var ch, start;
        hasLineTerminator = false;

        start = (index === 0);
        while (index < length) {
            ch = source.charCodeAt(index);

            if (isWhiteSpace(ch)) {
                ++index;
            } else if (isLineTerminator(ch)) {
                hasLineTerminator = true;
                ++index;
                if (ch === 0x0D && source.charCodeAt(index) === 0x0A) {
                    ++index;
                }
                ++lineNumber;
                lineStart = index;
                start = true;
            } else if (ch === 0x2F) { // U+002F is '/'
                ch = source.charCodeAt(index + 1);
                if (ch === 0x2F) {
                    ++index;
                    ++index;
                    skipSingleLineComment(2);
                    start = true;
                } else if (ch === 0x2A) {  // U+002A is '*'
                    ++index;
                    ++index;
                    skipMultiLineComment();
                } else {
                    break;
                }
            } else if (start && ch === 0x2D) { // U+002D is '-'
                // U+003E is '>'
                if ((source.charCodeAt(index + 1) === 0x2D) && (source.charCodeAt(index + 2) === 0x3E)) {
                    // '-->' is a single-line comment
                    index += 3;
                    skipSingleLineComment(3);
                } else {
                    break;
                }
            } else if (ch === 0x3C) { // U+003C is '<'
                if (source.slice(index + 1, index + 4) === '!--') {
                    ++index; // `<`
                    ++index; // `!`
                    ++index; // `-`
                    ++index; // `-`
                    skipSingleLineComment(4);
                } else {
                    break;
                }
            } else {
                break;
            }
        }
    }

    function scanHexEscape(prefix) {
        var i, len, ch, code = 0;

        len = (prefix === 'u') ? 4 : 2;
        for (i = 0; i < len; ++i) {
            if (index < length && isHexDigit(source[index])) {
                ch = source[index++];
                code = code * 16 + '0123456789abcdef'.indexOf(ch.toLowerCase());
            } else {
                return '';
            }
        }
        return String.fromCharCode(code);
    }

    function scanUnicodeCodePointEscape() {
        var ch, code, cu1, cu2;

        ch = source[index];
        code = 0;

        // At least, one hex digit is required.
        if (ch === '}') {
            throwUnexpectedToken();
        }

        while (index < length) {
            ch = source[index++];
            if (!isHexDigit(ch)) {
                break;
            }
            code = code * 16 + '0123456789abcdef'.indexOf(ch.toLowerCase());
        }

        if (code > 0x10FFFF || ch !== '}') {
            throwUnexpectedToken();
        }

        // UTF-16 Encoding
        if (code <= 0xFFFF) {
            return String.fromCharCode(code);
        }
        cu1 = ((code - 0x10000) >> 10) + 0xD800;
        cu2 = ((code - 0x10000) & 1023) + 0xDC00;
        return String.fromCharCode(cu1, cu2);
    }

    function getEscapedIdentifier() {
        var ch, id;

        ch = source.charCodeAt(index++);
        id = String.fromCharCode(ch);

        // '\u' (U+005C, U+0075) denotes an escaped character.
        if (ch === 0x5C) {
            if (source.charCodeAt(index) !== 0x75) {
                throwUnexpectedToken();
            }
            ++index;
            ch = scanHexEscape('u');
            if (!ch || ch === '\\' || !isIdentifierStart(ch.charCodeAt(0))) {
                throwUnexpectedToken();
            }
            id = ch;
        }

        while (index < length) {
            ch = source.charCodeAt(index);
            if (!isIdentifierPart(ch)) {
                break;
            }
            ++index;
            id += String.fromCharCode(ch);

            // '\u' (U+005C, U+0075) denotes an escaped character.
            if (ch === 0x5C) {
                id = id.substr(0, id.length - 1);
                if (source.charCodeAt(index) !== 0x75) {
                    throwUnexpectedToken();
                }
                ++index;
                ch = scanHexEscape('u');
                if (!ch || ch === '\\' || !isIdentifierPart(ch.charCodeAt(0))) {
                    throwUnexpectedToken();
                }
                id += ch;
            }
        }

        return id;
    }

    function getIdentifier() {
        var start, ch;

        start = index++;
        while (index < length) {
            ch = source.charCodeAt(index);
            if (ch === 0x5C) {
                // Blackslash (U+005C) marks Unicode escape sequence.
                index = start;
                return getEscapedIdentifier();
            }
            if (isIdentifierPart(ch)) {
                ++index;
            } else {
                break;
            }
        }

        return source.slice(start, index);
    }

    function scanIdentifier() {
        var start, id, type;

        start = index;

        // Backslash (U+005C) starts an escaped character.
        id = (source.charCodeAt(index) === 0x5C) ? getEscapedIdentifier() : getIdentifier();

        // There is no keyword or literal with only one character.
        // Thus, it must be an identifier.
        if (id.length === 1) {
            type = Token.Identifier;
        } else if (isKeyword(id)) {
            type = Token.Keyword;
        } else if (id === 'null') {
            type = Token.NullLiteral;
        } else if (id === 'true' || id === 'false') {
            type = Token.BooleanLiteral;
        } else {
            type = Token.Identifier;
        }

        return {
            type: type,
            value: id,
            lineNumber: lineNumber,
            lineStart: lineStart,
            start: start,
            end: index
        };
    }


    // 7.7 Punctuators

    function scanPunctuator() {
        var token, str;

        token = {
            type: Token.Punctuator,
            value: '',
            lineNumber: lineNumber,
            lineStart: lineStart,
            start: index,
            end: index
        };

        // Check for most common single-character punctuators.
        str = source[index];
        switch (str) {

        case '(':
            if (extra.tokenize) {
                extra.openParenToken = extra.tokens.length;
            }
            ++index;
            break;

        case '{':
            if (extra.tokenize) {
                extra.openCurlyToken = extra.tokens.length;
            }
            state.curlyStack.push('{');
            ++index;
            break;

        case '.':
            ++index;
            if (source[index] === '.' && source[index + 1] === '.') {
                // Spread operator: ...
                index += 2;
                str = '...';
            }
            break;

        case '}':
            ++index;
            state.curlyStack.pop();
            break;
        case ')':
        case ';':
        case ',':
        case '[':
        case ']':
        case ':':
        case '?':
        case '~':
            ++index;
            break;

        default:
            // 4-character punctuator.
            str = source.substr(index, 4);
            if (str === '>>>=') {
                index += 4;
            } else {

                // 3-character punctuators.
                str = str.substr(0, 3);
                if (str === '===' || str === '!==' || str === '>>>' ||
                    str === '<<=' || str === '>>=') {
                    index += 3;
                } else {

                    // 2-character punctuators.
                    str = str.substr(0, 2);
                    if (str === '&&' || str === '||' || str === '==' || str === '!=' ||
                        str === '+=' || str === '-=' || str === '*=' || str === '/=' ||
                        str === '++' || str === '--' || str === '<<' || str === '>>' ||
                        str === '&=' || str === '|=' || str === '^=' || str === '%=' ||
                        str === '<=' || str === '>=' || str === '=>') {
                        index += 2;
                    } else {

                        // 1-character punctuators.
                        str = source[index];
                        if ('<>=!+-*%&|^/'.indexOf(str) >= 0) {
                            ++index;
                        }
                    }
                }
            }
        }

        if (index === token.start) {
            throwUnexpectedToken();
        }

        token.end = index;
        token.value = str;
        return token;
    }

    // 7.8.3 Numeric Literals

    function scanHexLiteral(start) {
        var number = '';

        while (index < length) {
            if (!isHexDigit(source[index])) {
                break;
            }
            number += source[index++];
        }

        if (number.length === 0) {
            throwUnexpectedToken();
        }

        if (isIdentifierStart(source.charCodeAt(index))) {
            throwUnexpectedToken();
        }

        return {
            type: Token.NumericLiteral,
            value: parseInt('0x' + number, 16),
            lineNumber: lineNumber,
            lineStart: lineStart,
            start: start,
            end: index
        };
    }

    function scanBinaryLiteral(start) {
        var ch, number;

        number = '';

        while (index < length) {
            ch = source[index];
            if (ch !== '0' && ch !== '1') {
                break;
            }
            number += source[index++];
        }

        if (number.length === 0) {
            // only 0b or 0B
            throwUnexpectedToken();
        }

        if (index < length) {
            ch = source.charCodeAt(index);
            /* istanbul ignore else */
            if (isIdentifierStart(ch) || isDecimalDigit(ch)) {
                throwUnexpectedToken();
            }
        }

        return {
            type: Token.NumericLiteral,
            value: parseInt(number, 2),
            lineNumber: lineNumber,
            lineStart: lineStart,
            start: start,
            end: index
        };
    }

    function scanOctalLiteral(prefix, start) {
        var number, octal;

        if (isOctalDigit(prefix)) {
            octal = true;
            number = '0' + source[index++];
        } else {
            octal = false;
            ++index;
            number = '';
        }

        while (index < length) {
            if (!isOctalDigit(source[index])) {
                break;
            }
            number += source[index++];
        }

        if (!octal && number.length === 0) {
            // only 0o or 0O
            throwUnexpectedToken();
        }

        if (isIdentifierStart(source.charCodeAt(index)) || isDecimalDigit(source.charCodeAt(index))) {
            throwUnexpectedToken();
        }

        return {
            type: Token.NumericLiteral,
            value: parseInt(number, 8),
            octal: octal,
            lineNumber: lineNumber,
            lineStart: lineStart,
            start: start,
            end: index
        };
    }

    function isImplicitOctalLiteral() {
        var i, ch;

        // Implicit octal, unless there is a non-octal digit.
        // (Annex B.1.1 on Numeric Literals)
        for (i = index + 1; i < length; ++i) {
            ch = source[i];
            if (ch === '8' || ch === '9') {
                return false;
            }
            if (!isOctalDigit(ch)) {
                return true;
            }
        }

        return true;
    }

    function scanNumericLiteral() {
        var number, start, ch;

        ch = source[index];
        assert(isDecimalDigit(ch.charCodeAt(0)) || (ch === '.'),
            'Numeric literal must start with a decimal digit or a decimal point');

        start = index;
        number = '';
        if (ch !== '.') {
            number = source[index++];
            ch = source[index];

            // Hex number starts with '0x'.
            // Octal number starts with '0'.
            // Octal number in ES6 starts with '0o'.
            // Binary number in ES6 starts with '0b'.
            if (number === '0') {
                if (ch === 'x' || ch === 'X') {
                    ++index;
                    return scanHexLiteral(start);
                }
                if (ch === 'b' || ch === 'B') {
                    ++index;
                    return scanBinaryLiteral(start);
                }
                if (ch === 'o' || ch === 'O') {
                    return scanOctalLiteral(ch, start);
                }

                if (isOctalDigit(ch)) {
                    if (isImplicitOctalLiteral()) {
                        return scanOctalLiteral(ch, start);
                    }
                }
            }

            while (isDecimalDigit(source.charCodeAt(index))) {
                number += source[index++];
            }
            ch = source[index];
        }

        if (ch === '.') {
            number += source[index++];
            while (isDecimalDigit(source.charCodeAt(index))) {
                number += source[index++];
            }
            ch = source[index];
        }

        if (ch === 'e' || ch === 'E') {
            number += source[index++];

            ch = source[index];
            if (ch === '+' || ch === '-') {
                number += source[index++];
            }
            if (isDecimalDigit(source.charCodeAt(index))) {
                while (isDecimalDigit(source.charCodeAt(index))) {
                    number += source[index++];
                }
            } else {
                throwUnexpectedToken();
            }
        }

        if (isIdentifierStart(source.charCodeAt(index))) {
            throwUnexpectedToken();
        }

        return {
            type: Token.NumericLiteral,
            value: parseFloat(number),
            lineNumber: lineNumber,
            lineStart: lineStart,
            start: start,
            end: index
        };
    }

    // 7.8.4 String Literals

    function scanStringLiteral() {
        var str = '', quote, start, ch, unescaped, octToDec, octal = false;

        quote = source[index];
        assert((quote === '\'' || quote === '"'),
            'String literal must starts with a quote');

        start = index;
        ++index;

        while (index < length) {
            ch = source[index++];

            if (ch === quote) {
                quote = '';
                break;
            } else if (ch === '\\') {
                ch = source[index++];
                if (!ch || !isLineTerminator(ch.charCodeAt(0))) {
                    switch (ch) {
                    case 'u':
                    case 'x':
                        if (source[index] === '{') {
                            ++index;
                            str += scanUnicodeCodePointEscape();
                        } else {
                            unescaped = scanHexEscape(ch);
                            if (!unescaped) {
                                throw throwUnexpectedToken();
                            }
                            str += unescaped;
                        }
                        break;
                    case 'n':
                        str += '\n';
                        break;
                    case 'r':
                        str += '\r';
                        break;
                    case 't':
                        str += '\t';
                        break;
                    case 'b':
                        str += '\b';
                        break;
                    case 'f':
                        str += '\f';
                        break;
                    case 'v':
                        str += '\x0B';
                        break;
                    case '8':
                    case '9':
                        throw throwUnexpectedToken();

                    default:
                        if (isOctalDigit(ch)) {
                            octToDec = octalToDecimal(ch);

                            octal = octToDec.octal || octal;
                            str += String.fromCharCode(octToDec.code);
                        } else {
                            str += ch;
                        }
                        break;
                    }
                } else {
                    ++lineNumber;
                    if (ch === '\r' && source[index] === '\n') {
                        ++index;
                    }
                    lineStart = index;
                }
            } else if (isLineTerminator(ch.charCodeAt(0))) {
                break;
            } else {
                str += ch;
            }
        }

        if (quote !== '') {
            throwUnexpectedToken();
        }

        return {
            type: Token.StringLiteral,
            value: str,
            octal: octal,
            lineNumber: startLineNumber,
            lineStart: startLineStart,
            start: start,
            end: index
        };
    }

    function scanTemplate() {
        var cooked = '', ch, start, rawOffset, terminated, head, tail, restore, unescaped;

        terminated = false;
        tail = false;
        start = index;
        head = (source[index] === '`');
        rawOffset = 2;

        ++index;

        while (index < length) {
            ch = source[index++];
            if (ch === '`') {
                rawOffset = 1;
                tail = true;
                terminated = true;
                break;
            } else if (ch === '$') {
                if (source[index] === '{') {
                    state.curlyStack.push('${');
                    ++index;
                    terminated = true;
                    break;
                }
                cooked += ch;
            } else if (ch === '\\') {
                ch = source[index++];
                if (!isLineTerminator(ch.charCodeAt(0))) {
                    switch (ch) {
                    case 'n':
                        cooked += '\n';
                        break;
                    case 'r':
                        cooked += '\r';
                        break;
                    case 't':
                        cooked += '\t';
                        break;
                    case 'u':
                    case 'x':
                        if (source[index] === '{') {
                            ++index;
                            cooked += scanUnicodeCodePointEscape();
                        } else {
                            restore = index;
                            unescaped = scanHexEscape(ch);
                            if (unescaped) {
                                cooked += unescaped;
                            } else {
                                index = restore;
                                cooked += ch;
                            }
                        }
                        break;
                    case 'b':
                        cooked += '\b';
                        break;
                    case 'f':
                        cooked += '\f';
                        break;
                    case 'v':
                        cooked += '\v';
                        break;

                    default:
                        if (ch === '0') {
                            if (isDecimalDigit(source.charCodeAt(index))) {
                                // Illegal: \01 \02 and so on
                                throwError(Messages.TemplateOctalLiteral);
                            }
                            cooked += '\0';
                        } else if (isOctalDigit(ch)) {
                            // Illegal: \1 \2
                            throwError(Messages.TemplateOctalLiteral);
                        } else {
                            cooked += ch;
                        }
                        break;
                    }
                } else {
                    ++lineNumber;
                    if (ch === '\r' && source[index] === '\n') {
                        ++index;
                    }
                    lineStart = index;
                }
            } else if (isLineTerminator(ch.charCodeAt(0))) {
                ++lineNumber;
                if (ch === '\r' && source[index] === '\n') {
                    ++index;
                }
                lineStart = index;
                cooked += '\n';
            } else {
                cooked += ch;
            }
        }

        if (!terminated) {
            throwUnexpectedToken();
        }

        if (!head) {
            state.curlyStack.pop();
        }

        return {
            type: Token.Template,
            value: {
                cooked: cooked,
                raw: source.slice(start + 1, index - rawOffset)
            },
            head: head,
            tail: tail,
            lineNumber: lineNumber,
            lineStart: lineStart,
            start: start,
            end: index
        };
    }

    function testRegExp(pattern, flags) {
        // The BMP character to use as a replacement for astral symbols when
        // translating an ES6 "u"-flagged pattern to an ES5-compatible
        // approximation.
        // Note: replacing with '\uFFFF' enables false positives in unlikely
        // scenarios. For example, `[\u{1044f}-\u{10440}]` is an invalid
        // pattern that would not be detected by this substitution.
        var astralSubstitute = '\uFFFF',
            tmp = pattern;

        if (flags.indexOf('u') >= 0) {
            tmp = tmp
                // Replace every Unicode escape sequence with the equivalent
                // BMP character or a constant ASCII code point in the case of
                // astral symbols. (See the above note on `astralSubstitute`
                // for more information.)
                .replace(/\\u\{([0-9a-fA-F]+)\}|\\u([a-fA-F0-9]{4})/g, function ($0, $1, $2) {
                    var codePoint = parseInt($1 || $2, 16);
                    if (codePoint > 0x10FFFF) {
                        throwUnexpectedToken(null, Messages.InvalidRegExp);
                    }
                    if (codePoint <= 0xFFFF) {
                        return String.fromCharCode(codePoint);
                    }
                    return astralSubstitute;
                })
                // Replace each paired surrogate with a single ASCII symbol to
                // avoid throwing on regular expressions that are only valid in
                // combination with the "u" flag.
                .replace(
                    /[\uD800-\uDBFF][\uDC00-\uDFFF]/g,
                    astralSubstitute
                );
        }

        // First, detect invalid regular expressions.
        try {
            RegExp(tmp);
        } catch (e) {
            throwUnexpectedToken(null, Messages.InvalidRegExp);
        }

        // Return a regular expression object for this pattern-flag pair, or
        // `null` in case the current environment doesn't support the flags it
        // uses.
        try {
            return new RegExp(pattern, flags);
        } catch (exception) {
            return null;
        }
    }

    function scanRegExpBody() {
        var ch, str, classMarker, terminated, body;

        ch = source[index];
        assert(ch === '/', 'Regular expression literal must start with a slash');
        str = source[index++];

        classMarker = false;
        terminated = false;
        while (index < length) {
            ch = source[index++];
            str += ch;
            if (ch === '\\') {
                ch = source[index++];
                // ECMA-262 7.8.5
                if (isLineTerminator(ch.charCodeAt(0))) {
                    throwUnexpectedToken(null, Messages.UnterminatedRegExp);
                }
                str += ch;
            } else if (isLineTerminator(ch.charCodeAt(0))) {
                throwUnexpectedToken(null, Messages.UnterminatedRegExp);
            } else if (classMarker) {
                if (ch === ']') {
                    classMarker = false;
                }
            } else {
                if (ch === '/') {
                    terminated = true;
                    break;
                } else if (ch === '[') {
                    classMarker = true;
                }
            }
        }

        if (!terminated) {
            throwUnexpectedToken(null, Messages.UnterminatedRegExp);
        }

        // Exclude leading and trailing slash.
        body = str.substr(1, str.length - 2);
        return {
            value: body,
            literal: str
        };
    }

    function scanRegExpFlags() {
        var ch, str, flags, restore;

        str = '';
        flags = '';
        while (index < length) {
            ch = source[index];
            if (!isIdentifierPart(ch.charCodeAt(0))) {
                break;
            }

            ++index;
            if (ch === '\\' && index < length) {
                ch = source[index];
                if (ch === 'u') {
                    ++index;
                    restore = index;
                    ch = scanHexEscape('u');
                    if (ch) {
                        flags += ch;
                        for (str += '\\u'; restore < index; ++restore) {
                            str += source[restore];
                        }
                    } else {
                        index = restore;
                        flags += 'u';
                        str += '\\u';
                    }
                    tolerateUnexpectedToken();
                } else {
                    str += '\\';
                    tolerateUnexpectedToken();
                }
            } else {
                flags += ch;
                str += ch;
            }
        }

        return {
            value: flags,
            literal: str
        };
    }

    function scanRegExp() {
        scanning = true;
        var start, body, flags, value;

        lookahead = null;
        skipComment();
        start = index;

        body = scanRegExpBody();
        flags = scanRegExpFlags();
        value = testRegExp(body.value, flags.value);
        scanning = false;
        if (extra.tokenize) {
            return {
                type: Token.RegularExpression,
                value: value,
                regex: {
                    pattern: body.value,
                    flags: flags.value
                },
                lineNumber: lineNumber,
                lineStart: lineStart,
                start: start,
                end: index
            };
        }

        return {
            literal: body.literal + flags.literal,
            value: value,
            regex: {
                pattern: body.value,
                flags: flags.value
            },
            start: start,
            end: index
        };
    }

    function collectRegex() {
        var pos, loc, regex, token;

        skipComment();

        pos = index;
        loc = {
            start: {
                line: lineNumber,
                column: index - lineStart
            }
        };

        regex = scanRegExp();

        loc.end = {
            line: lineNumber,
            column: index - lineStart
        };

        /* istanbul ignore next */
        if (!extra.tokenize) {
            // Pop the previous token, which is likely '/' or '/='
            if (extra.tokens.length > 0) {
                token = extra.tokens[extra.tokens.length - 1];
                if (token.range[0] === pos && token.type === 'Punctuator') {
                    if (token.value === '/' || token.value === '/=') {
                        extra.tokens.pop();
                    }
                }
            }

            extra.tokens.push({
                type: 'RegularExpression',
                value: regex.literal,
                regex: regex.regex,
                range: [pos, index],
                loc: loc
            });
        }

        return regex;
    }

    function isIdentifierName(token) {
        return token.type === Token.Identifier ||
            token.type === Token.Keyword ||
            token.type === Token.BooleanLiteral ||
            token.type === Token.NullLiteral;
    }

    function advanceSlash() {
        var prevToken,
            checkToken;
        // Using the following algorithm:
        // https://github.com/mozilla/sweet.js/wiki/design
        prevToken = extra.tokens[extra.tokens.length - 1];
        if (!prevToken) {
            // Nothing before that: it cannot be a division.
            return collectRegex();
        }
        if (prevToken.type === 'Punctuator') {
            if (prevToken.value === ']') {
                return scanPunctuator();
            }
            if (prevToken.value === ')') {
                checkToken = extra.tokens[extra.openParenToken - 1];
                if (checkToken &&
                        checkToken.type === 'Keyword' &&
                        (checkToken.value === 'if' ||
                         checkToken.value === 'while' ||
                         checkToken.value === 'for' ||
                         checkToken.value === 'with')) {
                    return collectRegex();
                }
                return scanPunctuator();
            }
            if (prevToken.value === '}') {
                // Dividing a function by anything makes little sense,
                // but we have to check for that.
                if (extra.tokens[extra.openCurlyToken - 3] &&
                        extra.tokens[extra.openCurlyToken - 3].type === 'Keyword') {
                    // Anonymous function.
                    checkToken = extra.tokens[extra.openCurlyToken - 4];
                    if (!checkToken) {
                        return scanPunctuator();
                    }
                } else if (extra.tokens[extra.openCurlyToken - 4] &&
                        extra.tokens[extra.openCurlyToken - 4].type === 'Keyword') {
                    // Named function.
                    checkToken = extra.tokens[extra.openCurlyToken - 5];
                    if (!checkToken) {
                        return collectRegex();
                    }
                } else {
                    return scanPunctuator();
                }
                // checkToken determines whether the function is
                // a declaration or an expression.
                if (FnExprTokens.indexOf(checkToken.value) >= 0) {
                    // It is an expression.
                    return scanPunctuator();
                }
                // It is a declaration.
                return collectRegex();
            }
            return collectRegex();
        }
        if (prevToken.type === 'Keyword' && prevToken.value !== 'this') {
            return collectRegex();
        }
        return scanPunctuator();
    }

    function advance() {
        var ch, token;

        if (index >= length) {
            return {
                type: Token.EOF,
                lineNumber: lineNumber,
                lineStart: lineStart,
                start: index,
                end: index
            };
        }

        ch = source.charCodeAt(index);

        if (isIdentifierStart(ch)) {
            token = scanIdentifier();
            if (strict && isStrictModeReservedWord(token.value)) {
                token.type = Token.Keyword;
            }
            return token;
        }

        // Very common: ( and ) and ;
        if (ch === 0x28 || ch === 0x29 || ch === 0x3B) {
            return scanPunctuator();
        }

        // String literal starts with single quote (U+0027) or double quote (U+0022).
        if (ch === 0x27 || ch === 0x22) {
            return scanStringLiteral();
        }

        // Dot (.) U+002E can also start a floating-point number, hence the need
        // to check the next character.
        if (ch === 0x2E) {
            if (isDecimalDigit(source.charCodeAt(index + 1))) {
                return scanNumericLiteral();
            }
            return scanPunctuator();
        }

        if (isDecimalDigit(ch)) {
            return scanNumericLiteral();
        }

        // Slash (/) U+002F can also start a regex.
        if (extra.tokenize && ch === 0x2F) {
            return advanceSlash();
        }

        // Template literals start with ` (U+0060) for template head
        // or } (U+007D) for template middle or template tail.
        if (ch === 0x60 || (ch === 0x7D && state.curlyStack[state.curlyStack.length - 1] === '${')) {
            return scanTemplate();
        }

        return scanPunctuator();
    }

    function collectToken() {
        var loc, token, value, entry;

        loc = {
            start: {
                line: lineNumber,
                column: index - lineStart
            }
        };

        token = advance();
        loc.end = {
            line: lineNumber,
            column: index - lineStart
        };

        if (token.type !== Token.EOF) {
            value = source.slice(token.start, token.end);
            entry = {
                type: TokenName[token.type],
                value: value,
                range: [token.start, token.end],
                loc: loc
            };
            if (token.regex) {
                entry.regex = {
                    pattern: token.regex.pattern,
                    flags: token.regex.flags
                };
            }
            extra.tokens.push(entry);
        }

        return token;
    }

    function lex() {
        var token;
        scanning = true;

        lastIndex = index;
        lastLineNumber = lineNumber;
        lastLineStart = lineStart;

        skipComment();

        token = lookahead;

        startIndex = index;
        startLineNumber = lineNumber;
        startLineStart = lineStart;

        lookahead = (typeof extra.tokens !== 'undefined') ? collectToken() : advance();
        scanning = false;
        return token;
    }

    function peek() {
        scanning = true;

        skipComment();

        lastIndex = index;
        lastLineNumber = lineNumber;
        lastLineStart = lineStart;

        startIndex = index;
        startLineNumber = lineNumber;
        startLineStart = lineStart;

        lookahead = (typeof extra.tokens !== 'undefined') ? collectToken() : advance();
        scanning = false;
    }

    function Position() {
        this.line = startLineNumber;
        this.column = startIndex - startLineStart;
    }

    function SourceLocation() {
        this.start = new Position();
        this.end = null;
    }

    function WrappingSourceLocation(startToken) {
        this.start = {
            line: startToken.lineNumber,
            column: startToken.start - startToken.lineStart
        };
        this.end = null;
    }

    function Node() {
        if (extra.range) {
            this.range = [startIndex, 0];
        }
        if (extra.loc) {
            this.loc = new SourceLocation();
        }
    }

    function WrappingNode(startToken) {
        if (extra.range) {
            this.range = [startToken.start, 0];
        }
        if (extra.loc) {
            this.loc = new WrappingSourceLocation(startToken);
        }
    }

    WrappingNode.prototype = Node.prototype = {

        processComment: function () {
            var lastChild,
                leadingComments,
                trailingComments,
                bottomRight = extra.bottomRightStack,
                i,
                comment,
                last = bottomRight[bottomRight.length - 1];

            if (this.type === Syntax.Program) {
                if (this.body.length > 0) {
                    return;
                }
            }

            if (extra.trailingComments.length > 0) {
                trailingComments = [];
                for (i = extra.trailingComments.length - 1; i >= 0; --i) {
                    comment = extra.trailingComments[i];
                    if (comment.range[0] >= this.range[1]) {
                        trailingComments.unshift(comment);
                        extra.trailingComments.splice(i, 1);
                    }
                }
                extra.trailingComments = [];
            } else {
                if (last && last.trailingComments && last.trailingComments[0].range[0] >= this.range[1]) {
                    trailingComments = last.trailingComments;
                    delete last.trailingComments;
                }
            }

            // Eating the stack.
            while (last && last.range[0] >= this.range[0]) {
                lastChild = bottomRight.pop();
                last = bottomRight[bottomRight.length - 1];
            }

            if (lastChild) {
                if (lastChild.leadingComments) {
                    leadingComments = [];
                    for (i = lastChild.leadingComments.length - 1; i >= 0; --i) {
                        comment = lastChild.leadingComments[i];
                        if (comment.range[1] <= this.range[0]) {
                            leadingComments.unshift(comment);
                            lastChild.leadingComments.splice(i, 1);
                        }
                    }

                    if (!lastChild.leadingComments.length) {
                        lastChild.leadingComments = undefined;
                    }
                }
            } else if (extra.leadingComments.length > 0) {
                leadingComments = [];
                for (i = extra.leadingComments.length - 1; i >= 0; --i) {
                    comment = extra.leadingComments[i];
                    if (comment.range[1] <= this.range[0]) {
                        leadingComments.unshift(comment);
                        extra.leadingComments.splice(i, 1);
                    }
                }
            }


            if (leadingComments && leadingComments.length > 0) {
                this.leadingComments = leadingComments;
            }
            if (trailingComments && trailingComments.length > 0) {
                this.trailingComments = trailingComments;
            }

            bottomRight.push(this);
        },

        finish: function () {
            if (extra.range) {
                this.range[1] = lastIndex;
            }
            if (extra.loc) {
                this.loc.end = {
                    line: lastLineNumber,
                    column: lastIndex - lastLineStart
                };
                if (extra.source) {
                    this.loc.source = extra.source;
                }
            }

            if (extra.attachComment) {
                this.processComment();
            }
        },

        finishArrayExpression: function (elements) {
            this.type = Syntax.ArrayExpression;
            this.elements = elements;
            this.finish();
            return this;
        },

        finishArrayPattern: function (elements) {
            this.type = Syntax.ArrayPattern;
            this.elements = elements;
            this.finish();
            return this;
        },

        finishArrowFunctionExpression: function (params, defaults, body, expression) {
            this.type = Syntax.ArrowFunctionExpression;
            this.id = null;
            this.params = params;
            this.defaults = defaults;
            this.body = body;
            this.generator = false;
            this.expression = expression;
            this.finish();
            return this;
        },

        finishAssignmentExpression: function (operator, left, right) {
            this.type = Syntax.AssignmentExpression;
            this.operator = operator;
            this.left = left;
            this.right = right;
            this.finish();
            return this;
        },

        finishAssignmentPattern: function (left, right) {
            this.type = Syntax.AssignmentPattern;
            this.left = left;
            this.right = right;
            this.finish();
            return this;
        },

        finishBinaryExpression: function (operator, left, right) {
            this.type = (operator === '||' || operator === '&&') ? Syntax.LogicalExpression : Syntax.BinaryExpression;
            this.operator = operator;
            this.left = left;
            this.right = right;
            this.finish();
            return this;
        },

        finishBlockStatement: function (body) {
            this.type = Syntax.BlockStatement;
            this.body = body;
            this.finish();
            return this;
        },

        finishBreakStatement: function (label) {
            this.type = Syntax.BreakStatement;
            this.label = label;
            this.finish();
            return this;
        },

        finishCallExpression: function (callee, args) {
            this.type = Syntax.CallExpression;
            this.callee = callee;
            this.arguments = args;
            this.finish();
            return this;
        },

        finishCatchClause: function (param, body) {
            this.type = Syntax.CatchClause;
            this.param = param;
            this.body = body;
            this.finish();
            return this;
        },

        finishClassBody: function (body) {
            this.type = Syntax.ClassBody;
            this.body = body;
            this.finish();
            return this;
        },

        finishClassDeclaration: function (id, superClass, body) {
            this.type = Syntax.ClassDeclaration;
            this.id = id;
            this.superClass = superClass;
            this.body = body;
            this.finish();
            return this;
        },

        finishClassExpression: function (id, superClass, body) {
            this.type = Syntax.ClassExpression;
            this.id = id;
            this.superClass = superClass;
            this.body = body;
            this.finish();
            return this;
        },

        finishConditionalExpression: function (test, consequent, alternate) {
            this.type = Syntax.ConditionalExpression;
            this.test = test;
            this.consequent = consequent;
            this.alternate = alternate;
            this.finish();
            return this;
        },

        finishContinueStatement: function (label) {
            this.type = Syntax.ContinueStatement;
            this.label = label;
            this.finish();
            return this;
        },

        finishDebuggerStatement: function () {
            this.type = Syntax.DebuggerStatement;
            this.finish();
            return this;
        },

        finishDoWhileStatement: function (body, test) {
            this.type = Syntax.DoWhileStatement;
            this.body = body;
            this.test = test;
            this.finish();
            return this;
        },

        finishEmptyStatement: function () {
            this.type = Syntax.EmptyStatement;
            this.finish();
            return this;
        },

        finishExpressionStatement: function (expression) {
            this.type = Syntax.ExpressionStatement;
            this.expression = expression;
            this.finish();
            return this;
        },

        finishForStatement: function (init, test, update, body) {
            this.type = Syntax.ForStatement;
            this.init = init;
            this.test = test;
            this.update = update;
            this.body = body;
            this.finish();
            return this;
        },

        finishForOfStatement: function (left, right, body) {
            this.type = Syntax.ForOfStatement;
            this.left = left;
            this.right = right;
            this.body = body;
            this.finish();
            return this;
        },

        finishForInStatement: function (left, right, body) {
            this.type = Syntax.ForInStatement;
            this.left = left;
            this.right = right;
            this.body = body;
            this.each = false;
            this.finish();
            return this;
        },

        finishFunctionDeclaration: function (id, params, defaults, body, generator) {
            this.type = Syntax.FunctionDeclaration;
            this.id = id;
            this.params = params;
            this.defaults = defaults;
            this.body = body;
            this.generator = generator;
            this.expression = false;
            this.finish();
            return this;
        },

        finishFunctionExpression: function (id, params, defaults, body, generator) {
            this.type = Syntax.FunctionExpression;
            this.id = id;
            this.params = params;
            this.defaults = defaults;
            this.body = body;
            this.generator = generator;
            this.expression = false;
            this.finish();
            return this;
        },

        finishIdentifier: function (name) {
            this.type = Syntax.Identifier;
            this.name = name;
            this.finish();
            return this;
        },

        finishIfStatement: function (test, consequent, alternate) {
            this.type = Syntax.IfStatement;
            this.test = test;
            this.consequent = consequent;
            this.alternate = alternate;
            this.finish();
            return this;
        },

        finishLabeledStatement: function (label, body) {
            this.type = Syntax.LabeledStatement;
            this.label = label;
            this.body = body;
            this.finish();
            return this;
        },

        finishLiteral: function (token) {
            this.type = Syntax.Literal;
            this.value = token.value;
            this.raw = source.slice(token.start, token.end);
            if (token.regex) {
                this.regex = token.regex;
            }
            this.finish();
            return this;
        },

        finishMemberExpression: function (accessor, object, property) {
            this.type = Syntax.MemberExpression;
            this.computed = accessor === '[';
            this.object = object;
            this.property = property;
            this.finish();
            return this;
        },

        finishNewExpression: function (callee, args) {
            this.type = Syntax.NewExpression;
            this.callee = callee;
            this.arguments = args;
            this.finish();
            return this;
        },

        finishObjectExpression: function (properties) {
            this.type = Syntax.ObjectExpression;
            this.properties = properties;
            this.finish();
            return this;
        },

        finishObjectPattern: function (properties) {
            this.type = Syntax.ObjectPattern;
            this.properties = properties;
            this.finish();
            return this;
        },

        finishPostfixExpression: function (operator, argument) {
            this.type = Syntax.UpdateExpression;
            this.operator = operator;
            this.argument = argument;
            this.prefix = false;
            this.finish();
            return this;
        },

        finishProgram: function (body) {
            this.type = Syntax.Program;
            this.body = body;
            if (sourceType === 'module') {
                // very restrictive for now
                this.sourceType = sourceType;
            }
            this.finish();
            return this;
        },

        finishProperty: function (kind, key, computed, value, method, shorthand) {
            this.type = Syntax.Property;
            this.key = key;
            this.computed = computed;
            this.value = value;
            this.kind = kind;
            this.method = method;
            this.shorthand = shorthand;
            this.finish();
            return this;
        },

        finishRestElement: function (argument) {
            this.type = Syntax.RestElement;
            this.argument = argument;
            this.finish();
            return this;
        },

        finishReturnStatement: function (argument) {
            this.type = Syntax.ReturnStatement;
            this.argument = argument;
            this.finish();
            return this;
        },

        finishSequenceExpression: function (expressions) {
            this.type = Syntax.SequenceExpression;
            this.expressions = expressions;
            this.finish();
            return this;
        },

        finishSpreadElement: function (argument) {
            this.type = Syntax.SpreadElement;
            this.argument = argument;
            this.finish();
            return this;
        },

        finishSwitchCase: function (test, consequent) {
            this.type = Syntax.SwitchCase;
            this.test = test;
            this.consequent = consequent;
            this.finish();
            return this;
        },

        finishSuper: function () {
            this.type = Syntax.Super;
            this.finish();
            return this;
        },

        finishSwitchStatement: function (discriminant, cases) {
            this.type = Syntax.SwitchStatement;
            this.discriminant = discriminant;
            this.cases = cases;
            this.finish();
            return this;
        },

        finishTaggedTemplateExpression: function (tag, quasi) {
            this.type = Syntax.TaggedTemplateExpression;
            this.tag = tag;
            this.quasi = quasi;
            this.finish();
            return this;
        },

        finishTemplateElement: function (value, tail) {
            this.type = Syntax.TemplateElement;
            this.value = value;
            this.tail = tail;
            this.finish();
            return this;
        },

        finishTemplateLiteral: function (quasis, expressions) {
            this.type = Syntax.TemplateLiteral;
            this.quasis = quasis;
            this.expressions = expressions;
            this.finish();
            return this;
        },

        finishThisExpression: function () {
            this.type = Syntax.ThisExpression;
            this.finish();
            return this;
        },

        finishThrowStatement: function (argument) {
            this.type = Syntax.ThrowStatement;
            this.argument = argument;
            this.finish();
            return this;
        },

        finishTryStatement: function (block, handler, finalizer) {
            this.type = Syntax.TryStatement;
            this.block = block;
            this.guardedHandlers = [];
            this.handlers = handler ? [ handler ] : [];
            this.handler = handler;
            this.finalizer = finalizer;
            this.finish();
            return this;
        },

        finishUnaryExpression: function (operator, argument) {
            this.type = (operator === '++' || operator === '--') ? Syntax.UpdateExpression : Syntax.UnaryExpression;
            this.operator = operator;
            this.argument = argument;
            this.prefix = true;
            this.finish();
            return this;
        },

        finishVariableDeclaration: function (declarations) {
            this.type = Syntax.VariableDeclaration;
            this.declarations = declarations;
            this.kind = 'var';
            this.finish();
            return this;
        },

        finishLexicalDeclaration: function (declarations, kind) {
            this.type = Syntax.VariableDeclaration;
            this.declarations = declarations;
            this.kind = kind;
            this.finish();
            return this;
        },

        finishVariableDeclarator: function (id, init) {
            this.type = Syntax.VariableDeclarator;
            this.id = id;
            this.init = init;
            this.finish();
            return this;
        },

        finishWhileStatement: function (test, body) {
            this.type = Syntax.WhileStatement;
            this.test = test;
            this.body = body;
            this.finish();
            return this;
        },

        finishWithStatement: function (object, body) {
            this.type = Syntax.WithStatement;
            this.object = object;
            this.body = body;
            this.finish();
            return this;
        },

        finishExportSpecifier: function (local, exported) {
            this.type = Syntax.ExportSpecifier;
            this.exported = exported || local;
            this.local = local;
            this.finish();
            return this;
        },

        finishImportDefaultSpecifier: function (local) {
            this.type = Syntax.ImportDefaultSpecifier;
            this.local = local;
            this.finish();
            return this;
        },

        finishImportNamespaceSpecifier: function (local) {
            this.type = Syntax.ImportNamespaceSpecifier;
            this.local = local;
            this.finish();
            return this;
        },

        finishExportNamedDeclaration: function (declaration, specifiers, src) {
            this.type = Syntax.ExportNamedDeclaration;
            this.declaration = declaration;
            this.specifiers = specifiers;
            this.source = src;
            this.finish();
            return this;
        },

        finishExportDefaultDeclaration: function (declaration) {
            this.type = Syntax.ExportDefaultDeclaration;
            this.declaration = declaration;
            this.finish();
            return this;
        },

        finishExportAllDeclaration: function (src) {
            this.type = Syntax.ExportAllDeclaration;
            this.source = src;
            this.finish();
            return this;
        },

        finishImportSpecifier: function (local, imported) {
            this.type = Syntax.ImportSpecifier;
            this.local = local || imported;
            this.imported = imported;
            this.finish();
            return this;
        },

        finishImportDeclaration: function (specifiers, src) {
            this.type = Syntax.ImportDeclaration;
            this.specifiers = specifiers;
            this.source = src;
            this.finish();
            return this;
        },

        finishYieldExpression: function (argument, delegate) {
            this.type = Syntax.YieldExpression;
            this.argument = argument;
            this.delegate = delegate;
            this.finish();
            return this;
        }
    };


    function recordError(error) {
        var e, existing;

        for (e = 0; e < extra.errors.length; e++) {
            existing = extra.errors[e];
            // Prevent duplicated error.
            /* istanbul ignore next */
            if (existing.index === error.index && existing.message === error.message) {
                return;
            }
        }

        extra.errors.push(error);
    }

    function createError(line, pos, description) {
        var error = new Error('Line ' + line + ': ' + description);
        error.index = pos;
        error.lineNumber = line;
        error.column = pos - (scanning ? lineStart : lastLineStart) + 1;
        error.description = description;
        return error;
    }

    // Throw an exception

    function throwError(messageFormat) {
        var args, msg;

        args = Array.prototype.slice.call(arguments, 1);
        msg = messageFormat.replace(/%(\d)/g,
            function (whole, idx) {
                assert(idx < args.length, 'Message reference must be in range');
                return args[idx];
            }
        );

        throw createError(lastLineNumber, lastIndex, msg);
    }

    function tolerateError(messageFormat) {
        var args, msg, error;

        args = Array.prototype.slice.call(arguments, 1);
        /* istanbul ignore next */
        msg = messageFormat.replace(/%(\d)/g,
            function (whole, idx) {
                assert(idx < args.length, 'Message reference must be in range');
                return args[idx];
            }
        );

        error = createError(lineNumber, lastIndex, msg);
        if (extra.errors) {
            recordError(error);
        } else {
            throw error;
        }
    }

    // Throw an exception because of the token.

    function unexpectedTokenError(token, message) {
        var value, msg = message || Messages.UnexpectedToken;

        if (token) {
            if (!message) {
                msg = (token.type === Token.EOF) ? Messages.UnexpectedEOS :
                    (token.type === Token.Identifier) ? Messages.UnexpectedIdentifier :
                    (token.type === Token.NumericLiteral) ? Messages.UnexpectedNumber :
                    (token.type === Token.StringLiteral) ? Messages.UnexpectedString :
                    (token.type === Token.Template) ? Messages.UnexpectedTemplate :
                    Messages.UnexpectedToken;

                if (token.type === Token.Keyword) {
                    if (isFutureReservedWord(token.value)) {
                        msg = Messages.UnexpectedReserved;
                    } else if (strict && isStrictModeReservedWord(token.value)) {
                        msg = Messages.StrictReservedWord;
                    }
                }
            }

            value = (token.type === Token.Template) ? token.value.raw : token.value;
        } else {
            value = 'ILLEGAL';
        }

        msg = msg.replace('%0', value);

        return (token && typeof token.lineNumber === 'number') ?
            createError(token.lineNumber, token.start, msg) :
            createError(scanning ? lineNumber : lastLineNumber, scanning ? index : lastIndex, msg);
    }

    function throwUnexpectedToken(token, message) {
        throw unexpectedTokenError(token, message);
    }

    function tolerateUnexpectedToken(token, message) {
        var error = unexpectedTokenError(token, message);
        if (extra.errors) {
            recordError(error);
        } else {
            throw error;
        }
    }

    // Expect the next token to match the specified punctuator.
    // If not, an exception will be thrown.

    function expect(value) {
        var token = lex();
        if (token.type !== Token.Punctuator || token.value !== value) {
            throwUnexpectedToken(token);
        }
    }

    /**
     * @name expectCommaSeparator
     * @description Quietly expect a comma when in tolerant mode, otherwise delegates
     * to <code>expect(value)</code>
     * @since 2.0
     */
    function expectCommaSeparator() {
        var token;

        if (extra.errors) {
            token = lookahead;
            if (token.type === Token.Punctuator && token.value === ',') {
                lex();
            } else if (token.type === Token.Punctuator && token.value === ';') {
                lex();
                tolerateUnexpectedToken(token);
            } else {
                tolerateUnexpectedToken(token, Messages.UnexpectedToken);
            }
        } else {
            expect(',');
        }
    }

    // Expect the next token to match the specified keyword.
    // If not, an exception will be thrown.

    function expectKeyword(keyword) {
        var token = lex();
        if (token.type !== Token.Keyword || token.value !== keyword) {
            throwUnexpectedToken(token);
        }
    }

    // Return true if the next token matches the specified punctuator.

    function match(value) {
        return lookahead.type === Token.Punctuator && lookahead.value === value;
    }

    // Return true if the next token matches the specified keyword

    function matchKeyword(keyword) {
        return lookahead.type === Token.Keyword && lookahead.value === keyword;
    }

    // Return true if the next token matches the specified contextual keyword
    // (where an identifier is sometimes a keyword depending on the context)

    function matchContextualKeyword(keyword) {
        return lookahead.type === Token.Identifier && lookahead.value === keyword;
    }

    // Return true if the next token is an assignment operator

    function matchAssign() {
        var op;

        if (lookahead.type !== Token.Punctuator) {
            return false;
        }
        op = lookahead.value;
        return op === '=' ||
            op === '*=' ||
            op === '/=' ||
            op === '%=' ||
            op === '+=' ||
            op === '-=' ||
            op === '<<=' ||
            op === '>>=' ||
            op === '>>>=' ||
            op === '&=' ||
            op === '^=' ||
            op === '|=';
    }

    function consumeSemicolon() {
        // Catch the very common case first: immediately a semicolon (U+003B).
        if (source.charCodeAt(startIndex) === 0x3B || match(';')) {
            lex();
            return;
        }

        if (hasLineTerminator) {
            return;
        }

        // FIXME(ikarienator): this is seemingly an issue in the previous location info convention.
        lastIndex = startIndex;
        lastLineNumber = startLineNumber;
        lastLineStart = startLineStart;

        if (lookahead.type !== Token.EOF && !match('}')) {
            throwUnexpectedToken(lookahead);
        }
    }

    // Cover grammar support.
    //
    // When an assignment expression position starts with an left parenthesis, the determination of the type
    // of the syntax is to be deferred arbitrarily long until the end of the parentheses pair (plus a lookahead)
    // or the first comma. This situation also defers the determination of all the expressions nested in the pair.
    //
    // There are three productions that can be parsed in a parentheses pair that needs to be determined
    // after the outermost pair is closed. They are:
    //
    //   1. AssignmentExpression
    //   2. BindingElements
    //   3. AssignmentTargets
    //
    // In order to avoid exponential backtracking, we use two flags to denote if the production can be
    // binding element or assignment target.
    //
    // The three productions have the relationship:
    //
    //   BindingElements &sube; AssignmentTargets &sube; AssignmentExpression
    //
    // with a single exception that CoverInitializedName when used directly in an Expression, generates
    // an early error. Therefore, we need the third state, firstCoverInitializedNameError, to track the
    // first usage of CoverInitializedName and report it when we reached the end of the parentheses pair.
    //
    // isolateCoverGrammar function runs the given parser function with a new cover grammar context, and it does not
    // effect the current flags. This means the production the parser parses is only used as an expression. Therefore
    // the CoverInitializedName check is conducted.
    //
    // inheritCoverGrammar function runs the given parse function with a new cover grammar context, and it propagates
    // the flags outside of the parser. This means the production the parser parses is used as a part of a potential
    // pattern. The CoverInitializedName check is deferred.
    function isolateCoverGrammar(parser) {
        var oldIsBindingElement = isBindingElement,
            oldIsAssignmentTarget = isAssignmentTarget,
            oldFirstCoverInitializedNameError = firstCoverInitializedNameError,
            result;
        isBindingElement = true;
        isAssignmentTarget = true;
        firstCoverInitializedNameError = null;
        result = parser();
        if (firstCoverInitializedNameError !== null) {
            throwUnexpectedToken(firstCoverInitializedNameError);
        }
        isBindingElement = oldIsBindingElement;
        isAssignmentTarget = oldIsAssignmentTarget;
        firstCoverInitializedNameError = oldFirstCoverInitializedNameError;
        return result;
    }

    function inheritCoverGrammar(parser) {
        var oldIsBindingElement = isBindingElement,
            oldIsAssignmentTarget = isAssignmentTarget,
            oldFirstCoverInitializedNameError = firstCoverInitializedNameError,
            result;
        isBindingElement = true;
        isAssignmentTarget = true;
        firstCoverInitializedNameError = null;
        result = parser();
        isBindingElement = isBindingElement && oldIsBindingElement;
        isAssignmentTarget = isAssignmentTarget && oldIsAssignmentTarget;
        firstCoverInitializedNameError = oldFirstCoverInitializedNameError || firstCoverInitializedNameError;
        return result;
    }

    function parseArrayPattern() {
        var node = new Node(), elements = [], rest, restNode;
        expect('[');

        while (!match(']')) {
            if (match(',')) {
                lex();
                elements.push(null);
            } else {
                if (match('...')) {
                    restNode = new Node();
                    lex();
                    rest = parseVariableIdentifier();
                    elements.push(restNode.finishRestElement(rest));
                    break;
                } else {
                    elements.push(parsePatternWithDefault());
                }
                if (!match(']')) {
                    expect(',');
                }
            }

        }

        expect(']');

        return node.finishArrayPattern(elements);
    }

    function parsePropertyPattern() {
        var node = new Node(), key, computed = match('['), init;
        if (lookahead.type === Token.Identifier) {
            key = parseVariableIdentifier();
            if (match('=')) {
                lex();
                init = parseAssignmentExpression();

                return node.finishProperty(
                    'init', key, false,
                    new WrappingNode(key).finishAssignmentPattern(key, init), false, false);
            } else if (!match(':')) {
                return node.finishProperty('init', key, false, key, false, true);
            }
        } else {
            key = parseObjectPropertyKey();
        }
        expect(':');
        init = parsePatternWithDefault();
        return node.finishProperty('init', key, computed, init, false, false);
    }

    function parseObjectPattern() {
        var node = new Node(), properties = [];

        expect('{');

        while (!match('}')) {
            properties.push(parsePropertyPattern());
            if (!match('}')) {
                expect(',');
            }
        }

        lex();

        return node.finishObjectPattern(properties);
    }

    function parsePattern() {
        if (lookahead.type === Token.Identifier) {
            return parseVariableIdentifier();
        } else if (match('[')) {
            return parseArrayPattern();
        } else if (match('{')) {
            return parseObjectPattern();
        }
        throwUnexpectedToken(lookahead);
    }

    function parsePatternWithDefault() {
        var startToken = lookahead, pattern, right;
        pattern = parsePattern();
        if (match('=')) {
            lex();
            right = isolateCoverGrammar(parseAssignmentExpression);
            pattern = new WrappingNode(startToken).finishAssignmentPattern(pattern, right);
        }
        return pattern;
    }

    // 11.1.4 Array Initialiser

    function parseArrayInitialiser() {
        var elements = [], node = new Node(), restSpread;

        expect('[');

        while (!match(']')) {
            if (match(',')) {
                lex();
                elements.push(null);
            } else if (match('...')) {
                restSpread = new Node();
                lex();
                restSpread.finishSpreadElement(inheritCoverGrammar(parseAssignmentExpression));

                if (!match(']')) {
                    isAssignmentTarget = isBindingElement = false;
                    expect(',');
                }
                elements.push(restSpread);
            } else {
                elements.push(inheritCoverGrammar(parseAssignmentExpression));

                if (!match(']')) {
                    expect(',');
                }
            }
        }

        lex();

        return node.finishArrayExpression(elements);
    }

    // 11.1.5 Object Initialiser

    function parsePropertyFunction(node, paramInfo, isGenerator) {
        var previousStrict, body;

        isAssignmentTarget = isBindingElement = false;

        previousStrict = strict;
        body = isolateCoverGrammar(parseFunctionSourceElements);

        if (strict && paramInfo.firstRestricted) {
            tolerateUnexpectedToken(paramInfo.firstRestricted, paramInfo.message);
        }
        if (strict && paramInfo.stricted) {
            tolerateUnexpectedToken(paramInfo.stricted, paramInfo.message);
        }

        strict = previousStrict;
        return node.finishFunctionExpression(null, paramInfo.params, paramInfo.defaults, body, isGenerator);
    }

    function parsePropertyMethodFunction() {
        var params, method, node = new Node(),
            previousAllowYield = state.allowYield;

        state.allowYield = false;
        params = parseParams();
        state.allowYield = previousAllowYield;

        state.allowYield = false;
        method = parsePropertyFunction(node, params, false);
        state.allowYield = previousAllowYield;

        return method;
    }

    function parseObjectPropertyKey() {
        var token, node = new Node(), expr;

        token = lex();

        // Note: This function is called only from parseObjectProperty(), where
        // EOF and Punctuator tokens are already filtered out.

        switch (token.type) {
        case Token.StringLiteral:
        case Token.NumericLiteral:
            if (strict && token.octal) {
                tolerateUnexpectedToken(token, Messages.StrictOctalLiteral);
            }
            return node.finishLiteral(token);
        case Token.Identifier:
        case Token.BooleanLiteral:
        case Token.NullLiteral:
        case Token.Keyword:
            return node.finishIdentifier(token.value);
        case Token.Punctuator:
            if (token.value === '[') {
                expr = isolateCoverGrammar(parseAssignmentExpression);
                expect(']');
                return expr;
            }
            break;
        }
        throwUnexpectedToken(token);
    }

    function lookaheadPropertyName() {
        switch (lookahead.type) {
        case Token.Identifier:
        case Token.StringLiteral:
        case Token.BooleanLiteral:
        case Token.NullLiteral:
        case Token.NumericLiteral:
        case Token.Keyword:
            return true;
        case Token.Punctuator:
            return lookahead.value === '[';
        }
        return false;
    }

    // This function is to try to parse a MethodDefinition as defined in 14.3. But in the case of object literals,
    // it might be called at a position where there is in fact a short hand identifier pattern or a data property.
    // This can only be determined after we consumed up to the left parentheses.
    //
    // In order to avoid back tracking, it returns `null` if the position is not a MethodDefinition and the caller
    // is responsible to visit other options.
    function tryParseMethodDefinition(token, key, computed, node) {
        var value, options, methodNode, params,
            previousAllowYield = state.allowYield;

        if (token.type === Token.Identifier) {
            // check for `get` and `set`;

            if (token.value === 'get' && lookaheadPropertyName()) {
                computed = match('[');
                key = parseObjectPropertyKey();
                methodNode = new Node();
                expect('(');
                expect(')');

                state.allowYield = false;
                value = parsePropertyFunction(methodNode, {
                    params: [],
                    defaults: [],
                    stricted: null,
                    firstRestricted: null,
                    message: null
                }, false);
                state.allowYield = previousAllowYield;

                return node.finishProperty('get', key, computed, value, false, false);
            } else if (token.value === 'set' && lookaheadPropertyName()) {
                computed = match('[');
                key = parseObjectPropertyKey();
                methodNode = new Node();
                expect('(');

                options = {
                    params: [],
                    defaultCount: 0,
                    defaults: [],
                    firstRestricted: null,
                    paramSet: {}
                };
                if (match(')')) {
                    tolerateUnexpectedToken(lookahead);
                } else {
                    state.allowYield = false;
                    parseParam(options);
                    state.allowYield = previousAllowYield;
                    if (options.defaultCount === 0) {
                        options.defaults = [];
                    }
                }
                expect(')');

                state.allowYield = false;
                value = parsePropertyFunction(methodNode, options, false);
                state.allowYield = previousAllowYield;

                return node.finishProperty('set', key, computed, value, false, false);
            }
        } else if (token.type === Token.Punctuator && token.value === '*' && lookaheadPropertyName()) {
            computed = match('[');
            key = parseObjectPropertyKey();
            methodNode = new Node();

            state.allowYield = false;
            params = parseParams();
            state.allowYield = previousAllowYield;

            state.allowYield = true;
            value = parsePropertyFunction(methodNode, params, true);
            state.allowYield = previousAllowYield;

            return node.finishProperty('init', key, computed, value, true, false);
        }

        if (key && match('(')) {
            value = parsePropertyMethodFunction();
            return node.finishProperty('init', key, computed, value, true, false);
        }

        // Not a MethodDefinition.
        return null;
    }

    function checkProto(key, computed, hasProto) {
        if (computed === false && (key.type === Syntax.Identifier && key.name === '__proto__' ||
            key.type === Syntax.Literal && key.value === '__proto__')) {
            if (hasProto.value) {
                tolerateError(Messages.DuplicateProtoProperty);
            } else {
                hasProto.value = true;
            }
        }
    }

    function parseObjectProperty(hasProto) {
        var token = lookahead, node = new Node(), computed, key, maybeMethod, value;

        computed = match('[');
        if (match('*')) {
            lex();
        } else {
            key = parseObjectPropertyKey();
        }
        maybeMethod = tryParseMethodDefinition(token, key, computed, node);

        if (maybeMethod) {
            checkProto(maybeMethod.key, maybeMethod.computed, hasProto);
            // finished
            return maybeMethod;
        }

        if (!key) {
            throwUnexpectedToken(lookahead);
        }

        // init property or short hand property.
        checkProto(key, computed, hasProto);

        if (match(':')) {
            lex();
            value = inheritCoverGrammar(parseAssignmentExpression);
            return node.finishProperty('init', key, computed, value, false, false);
        }

        if (token.type === Token.Identifier) {
            if (match('=')) {
                firstCoverInitializedNameError = lookahead;
                lex();
                value = isolateCoverGrammar(parseAssignmentExpression);
                return node.finishProperty('init', key, computed,
                    new WrappingNode(token).finishAssignmentPattern(key, value), false, true);
            }
            return node.finishProperty('init', key, computed, key, false, true);
        }

        throwUnexpectedToken(lookahead);
    }

    function parseObjectInitialiser() {
        var properties = [], hasProto = {value: false}, node = new Node();

        expect('{');

        while (!match('}')) {
            properties.push(parseObjectProperty(hasProto));

            if (!match('}')) {
                expectCommaSeparator();
            }
        }

        expect('}');

        return node.finishObjectExpression(properties);
    }

    function reinterpretExpressionAsPattern(expr) {
        var i;
        switch (expr.type) {
        case Syntax.Identifier:
        case Syntax.MemberExpression:
        case Syntax.RestElement:
        case Syntax.AssignmentPattern:
            break;
        case Syntax.SpreadElement:
            expr.type = Syntax.RestElement;
            reinterpretExpressionAsPattern(expr.argument);
            break;
        case Syntax.ArrayExpression:
            expr.type = Syntax.ArrayPattern;
            for (i = 0; i < expr.elements.length; i++) {
                if (expr.elements[i] !== null) {
                    reinterpretExpressionAsPattern(expr.elements[i]);
                }
            }
            break;
        case Syntax.ObjectExpression:
            expr.type = Syntax.ObjectPattern;
            for (i = 0; i < expr.properties.length; i++) {
                reinterpretExpressionAsPattern(expr.properties[i].value);
            }
            break;
        case Syntax.AssignmentExpression:
            expr.type = Syntax.AssignmentPattern;
            reinterpretExpressionAsPattern(expr.left);
            break;
        default:
            // Allow other node type for tolerant parsing.
            break;
        }
    }

    function parseTemplateElement(option) {
        var node, token;

        if (lookahead.type !== Token.Template || (option.head && !lookahead.head)) {
            throwUnexpectedToken();
        }

        node = new Node();
        token = lex();

        return node.finishTemplateElement({ raw: token.value.raw, cooked: token.value.cooked }, token.tail);
    }

    function parseTemplateLiteral() {
        var quasi, quasis, expressions, node = new Node();

        quasi = parseTemplateElement({ head: true });
        quasis = [ quasi ];
        expressions = [];

        while (!quasi.tail) {
            expressions.push(parseExpression());
            quasi = parseTemplateElement({ head: false });
            quasis.push(quasi);
        }

        return node.finishTemplateLiteral(quasis, expressions);
    }

    // 11.1.6 The Grouping Operator

    function parseGroupExpression() {
        var expr, expressions, startToken, i;

        expect('(');

        if (match(')')) {
            lex();
            if (!match('=>')) {
                expect('=>');
            }
            return {
                type: PlaceHolders.ArrowParameterPlaceHolder,
                params: []
            };
        }

        startToken = lookahead;
        if (match('...')) {
            expr = parseRestElement();
            expect(')');
            if (!match('=>')) {
                expect('=>');
            }
            return {
                type: PlaceHolders.ArrowParameterPlaceHolder,
                params: [expr]
            };
        }

        isBindingElement = true;
        expr = inheritCoverGrammar(parseAssignmentExpression);

        if (match(',')) {
            isAssignmentTarget = false;
            expressions = [expr];

            while (startIndex < length) {
                if (!match(',')) {
                    break;
                }
                lex();

                if (match('...')) {
                    if (!isBindingElement) {
                        throwUnexpectedToken(lookahead);
                    }
                    expressions.push(parseRestElement());
                    expect(')');
                    if (!match('=>')) {
                        expect('=>');
                    }
                    isBindingElement = false;
                    for (i = 0; i < expressions.length; i++) {
                        reinterpretExpressionAsPattern(expressions[i]);
                    }
                    return {
                        type: PlaceHolders.ArrowParameterPlaceHolder,
                        params: expressions
                    };
                }

                expressions.push(inheritCoverGrammar(parseAssignmentExpression));
            }

            expr = new WrappingNode(startToken).finishSequenceExpression(expressions);
        }


        expect(')');

        if (match('=>')) {
            if (!isBindingElement) {
                throwUnexpectedToken(lookahead);
            }

            if (expr.type === Syntax.SequenceExpression) {
                for (i = 0; i < expr.expressions.length; i++) {
                    reinterpretExpressionAsPattern(expr.expressions[i]);
                }
            } else {
                reinterpretExpressionAsPattern(expr);
            }

            expr = {
                type: PlaceHolders.ArrowParameterPlaceHolder,
                params: expr.type === Syntax.SequenceExpression ? expr.expressions : [expr]
            };
        }
        isBindingElement = false;
        return expr;
    }


    // 11.1 Primary Expressions

    function parsePrimaryExpression() {
        var type, token, expr, node;

        if (match('(')) {
            isBindingElement = false;
            return inheritCoverGrammar(parseGroupExpression);
        }

        if (match('[')) {
            return inheritCoverGrammar(parseArrayInitialiser);
        }

        if (match('{')) {
            return inheritCoverGrammar(parseObjectInitialiser);
        }

        type = lookahead.type;
        node = new Node();

        if (type === Token.Identifier) {
            expr = node.finishIdentifier(lex().value);
        } else if (type === Token.StringLiteral || type === Token.NumericLiteral) {
            isAssignmentTarget = isBindingElement = false;
            if (strict && lookahead.octal) {
                tolerateUnexpectedToken(lookahead, Messages.StrictOctalLiteral);
            }
            expr = node.finishLiteral(lex());
        } else if (type === Token.Keyword) {
            isAssignmentTarget = isBindingElement = false;
            if (matchKeyword('function')) {
                return parseFunctionExpression();
            }
            if (matchKeyword('this')) {
                lex();
                return node.finishThisExpression();
            }
            if (matchKeyword('class')) {
                return parseClassExpression();
            }
            throwUnexpectedToken(lex());
        } else if (type === Token.BooleanLiteral) {
            isAssignmentTarget = isBindingElement = false;
            token = lex();
            token.value = (token.value === 'true');
            expr = node.finishLiteral(token);
        } else if (type === Token.NullLiteral) {
            isAssignmentTarget = isBindingElement = false;
            token = lex();
            token.value = null;
            expr = node.finishLiteral(token);
        } else if (match('/') || match('/=')) {
            isAssignmentTarget = isBindingElement = false;
            index = startIndex;

            if (typeof extra.tokens !== 'undefined') {
                token = collectRegex();
            } else {
                token = scanRegExp();
            }
            lex();
            expr = node.finishLiteral(token);
        } else if (type === Token.Template) {
            expr = parseTemplateLiteral();
        } else {
            throwUnexpectedToken(lex());
        }

        return expr;
    }

    // 11.2 Left-Hand-Side Expressions

    function parseArguments() {
        var args = [], expr;

        expect('(');

        if (!match(')')) {
            while (startIndex < length) {
                if (match('...')) {
                    expr = new Node();
                    lex();
                    expr.finishSpreadElement(isolateCoverGrammar(parseAssignmentExpression));
                } else {
                    expr = isolateCoverGrammar(parseAssignmentExpression);
                }
                args.push(expr);
                if (match(')')) {
                    break;
                }
                expectCommaSeparator();
            }
        }

        expect(')');

        return args;
    }

    function parseNonComputedProperty() {
        var token, node = new Node();

        token = lex();

        if (!isIdentifierName(token)) {
            throwUnexpectedToken(token);
        }

        return node.finishIdentifier(token.value);
    }

    function parseNonComputedMember() {
        expect('.');

        return parseNonComputedProperty();
    }

    function parseComputedMember() {
        var expr;

        expect('[');

        expr = isolateCoverGrammar(parseExpression);

        expect(']');

        return expr;
    }

    function parseNewExpression() {
        var callee, args, node = new Node();

        expectKeyword('new');
        callee = isolateCoverGrammar(parseLeftHandSideExpression);
        args = match('(') ? parseArguments() : [];

        isAssignmentTarget = isBindingElement = false;

        return node.finishNewExpression(callee, args);
    }

    function parseLeftHandSideExpressionAllowCall() {
        var quasi, expr, args, property, startToken, previousAllowIn = state.allowIn;

        startToken = lookahead;
        state.allowIn = true;

        if (matchKeyword('super') && state.inFunctionBody) {
            expr = new Node();
            lex();
            expr = expr.finishSuper();
            if (!match('(') && !match('.') && !match('[')) {
                throwUnexpectedToken(lookahead);
            }
        } else {
            expr = inheritCoverGrammar(matchKeyword('new') ? parseNewExpression : parsePrimaryExpression);
        }

        for (;;) {
            if (match('.')) {
                isBindingElement = false;
                isAssignmentTarget = true;
                property = parseNonComputedMember();
                expr = new WrappingNode(startToken).finishMemberExpression('.', expr, property);
            } else if (match('(')) {
                isBindingElement = false;
                isAssignmentTarget = false;
                args = parseArguments();
                expr = new WrappingNode(startToken).finishCallExpression(expr, args);
            } else if (match('[')) {
                isBindingElement = false;
                isAssignmentTarget = true;
                property = parseComputedMember();
                expr = new WrappingNode(startToken).finishMemberExpression('[', expr, property);
            } else if (lookahead.type === Token.Template && lookahead.head) {
                quasi = parseTemplateLiteral();
                expr = new WrappingNode(startToken).finishTaggedTemplateExpression(expr, quasi);
            } else {
                break;
            }
        }
        state.allowIn = previousAllowIn;

        return expr;
    }

    function parseLeftHandSideExpression() {
        var quasi, expr, property, startToken;
        assert(state.allowIn, 'callee of new expression always allow in keyword.');

        startToken = lookahead;

        if (matchKeyword('super') && state.inFunctionBody) {
            expr = new Node();
            lex();
            expr = expr.finishSuper();
            if (!match('[') && !match('.')) {
                throwUnexpectedToken(lookahead);
            }
        } else {
            expr = inheritCoverGrammar(matchKeyword('new') ? parseNewExpression : parsePrimaryExpression);
        }

        for (;;) {
            if (match('[')) {
                isBindingElement = false;
                isAssignmentTarget = true;
                property = parseComputedMember();
                expr = new WrappingNode(startToken).finishMemberExpression('[', expr, property);
            } else if (match('.')) {
                isBindingElement = false;
                isAssignmentTarget = true;
                property = parseNonComputedMember();
                expr = new WrappingNode(startToken).finishMemberExpression('.', expr, property);
            } else if (lookahead.type === Token.Template && lookahead.head) {
                quasi = parseTemplateLiteral();
                expr = new WrappingNode(startToken).finishTaggedTemplateExpression(expr, quasi);
            } else {
                break;
            }
        }
        return expr;
    }

    // 11.3 Postfix Expressions

    function parsePostfixExpression() {
        var expr, token, startToken = lookahead;

        expr = inheritCoverGrammar(parseLeftHandSideExpressionAllowCall);

        if (!hasLineTerminator && lookahead.type === Token.Punctuator) {
            if (match('++') || match('--')) {
                // 11.3.1, 11.3.2
                if (strict && expr.type === Syntax.Identifier && isRestrictedWord(expr.name)) {
                    tolerateError(Messages.StrictLHSPostfix);
                }

                if (!isAssignmentTarget) {
                    tolerateError(Messages.InvalidLHSInAssignment);
                }

                isAssignmentTarget = isBindingElement = false;

                token = lex();
                expr = new WrappingNode(startToken).finishPostfixExpression(token.value, expr);
            }
        }

        return expr;
    }

    // 11.4 Unary Operators

    function parseUnaryExpression() {
        var token, expr, startToken;

        if (lookahead.type !== Token.Punctuator && lookahead.type !== Token.Keyword) {
            expr = parsePostfixExpression();
        } else if (match('++') || match('--')) {
            startToken = lookahead;
            token = lex();
            expr = inheritCoverGrammar(parseUnaryExpression);
            // 11.4.4, 11.4.5
            if (strict && expr.type === Syntax.Identifier && isRestrictedWord(expr.name)) {
                tolerateError(Messages.StrictLHSPrefix);
            }

            if (!isAssignmentTarget) {
                tolerateError(Messages.InvalidLHSInAssignment);
            }
            expr = new WrappingNode(startToken).finishUnaryExpression(token.value, expr);
            isAssignmentTarget = isBindingElement = false;
        } else if (match('+') || match('-') || match('~') || match('!')) {
            startToken = lookahead;
            token = lex();
            expr = inheritCoverGrammar(parseUnaryExpression);
            expr = new WrappingNode(startToken).finishUnaryExpression(token.value, expr);
            isAssignmentTarget = isBindingElement = false;
        } else if (matchKeyword('delete') || matchKeyword('void') || matchKeyword('typeof')) {
            startToken = lookahead;
            token = lex();
            expr = inheritCoverGrammar(parseUnaryExpression);
            expr = new WrappingNode(startToken).finishUnaryExpression(token.value, expr);
            if (strict && expr.operator === 'delete' && expr.argument.type === Syntax.Identifier) {
                tolerateError(Messages.StrictDelete);
            }
            isAssignmentTarget = isBindingElement = false;
        } else {
            expr = parsePostfixExpression();
        }

        return expr;
    }

    function binaryPrecedence(token, allowIn) {
        var prec = 0;

        if (token.type !== Token.Punctuator && token.type !== Token.Keyword) {
            return 0;
        }

        switch (token.value) {
        case '||':
            prec = 1;
            break;

        case '&&':
            prec = 2;
            break;

        case '|':
            prec = 3;
            break;

        case '^':
            prec = 4;
            break;

        case '&':
            prec = 5;
            break;

        case '==':
        case '!=':
        case '===':
        case '!==':
            prec = 6;
            break;

        case '<':
        case '>':
        case '<=':
        case '>=':
        case 'instanceof':
            prec = 7;
            break;

        case 'in':
            prec = allowIn ? 7 : 0;
            break;

        case '<<':
        case '>>':
        case '>>>':
            prec = 8;
            break;

        case '+':
        case '-':
            prec = 9;
            break;

        case '*':
        case '/':
        case '%':
            prec = 11;
            break;

        default:
            break;
        }

        return prec;
    }

    // 11.5 Multiplicative Operators
    // 11.6 Additive Operators
    // 11.7 Bitwise Shift Operators
    // 11.8 Relational Operators
    // 11.9 Equality Operators
    // 11.10 Binary Bitwise Operators
    // 11.11 Binary Logical Operators

    function parseBinaryExpression() {
        var marker, markers, expr, token, prec, stack, right, operator, left, i;

        marker = lookahead;
        left = inheritCoverGrammar(parseUnaryExpression);

        token = lookahead;
        prec = binaryPrecedence(token, state.allowIn);
        if (prec === 0) {
            return left;
        }
        isAssignmentTarget = isBindingElement = false;
        token.prec = prec;
        lex();

        markers = [marker, lookahead];
        right = isolateCoverGrammar(parseUnaryExpression);

        stack = [left, token, right];

        while ((prec = binaryPrecedence(lookahead, state.allowIn)) > 0) {

            // Reduce: make a binary expression from the three topmost entries.
            while ((stack.length > 2) && (prec <= stack[stack.length - 2].prec)) {
                right = stack.pop();
                operator = stack.pop().value;
                left = stack.pop();
                markers.pop();
                expr = new WrappingNode(markers[markers.length - 1]).finishBinaryExpression(operator, left, right);
                stack.push(expr);
            }

            // Shift.
            token = lex();
            token.prec = prec;
            stack.push(token);
            markers.push(lookahead);
            expr = isolateCoverGrammar(parseUnaryExpression);
            stack.push(expr);
        }

        // Final reduce to clean-up the stack.
        i = stack.length - 1;
        expr = stack[i];
        markers.pop();
        while (i > 1) {
            expr = new WrappingNode(markers.pop()).finishBinaryExpression(stack[i - 1].value, stack[i - 2], expr);
            i -= 2;
        }

        return expr;
    }


    // 11.12 Conditional Operator

    function parseConditionalExpression() {
        var expr, previousAllowIn, consequent, alternate, startToken;

        startToken = lookahead;

        expr = inheritCoverGrammar(parseBinaryExpression);
        if (match('?')) {
            lex();
            previousAllowIn = state.allowIn;
            state.allowIn = true;
            consequent = isolateCoverGrammar(parseAssignmentExpression);
            state.allowIn = previousAllowIn;
            expect(':');
            alternate = isolateCoverGrammar(parseAssignmentExpression);

            expr = new WrappingNode(startToken).finishConditionalExpression(expr, consequent, alternate);
            isAssignmentTarget = isBindingElement = false;
        }

        return expr;
    }

    // [ES6] 14.2 Arrow Function

    function parseConciseBody() {
        if (match('{')) {
            return parseFunctionSourceElements();
        }
        return isolateCoverGrammar(parseAssignmentExpression);
    }

    function checkPatternParam(options, param) {
        var i;
        switch (param.type) {
        case Syntax.Identifier:
            validateParam(options, param, param.name);
            break;
        case Syntax.RestElement:
            checkPatternParam(options, param.argument);
            break;
        case Syntax.AssignmentPattern:
            checkPatternParam(options, param.left);
            break;
        case Syntax.ArrayPattern:
            for (i = 0; i < param.elements.length; i++) {
                if (param.elements[i] !== null) {
                    checkPatternParam(options, param.elements[i]);
                }
            }
            break;
        default:
            assert(param.type === Syntax.ObjectPattern, 'Invalid type');
            for (i = 0; i < param.properties.length; i++) {
                checkPatternParam(options, param.properties[i].value);
            }
            break;
        }
    }
    function reinterpretAsCoverFormalsList(expr) {
        var i, len, param, params, defaults, defaultCount, options, token;

        defaults = [];
        defaultCount = 0;
        params = [expr];

        switch (expr.type) {
        case Syntax.Identifier:
            break;
        case PlaceHolders.ArrowParameterPlaceHolder:
            params = expr.params;
            break;
        default:
            return null;
        }

        options = {
            paramSet: {}
        };

        for (i = 0, len = params.length; i < len; i += 1) {
            param = params[i];
            switch (param.type) {
            case Syntax.AssignmentPattern:
                params[i] = param.left;
                defaults.push(param.right);
                ++defaultCount;
                checkPatternParam(options, param.left);
                break;
            default:
                checkPatternParam(options, param);
                params[i] = param;
                defaults.push(null);
                break;
            }
        }

        if (options.message === Messages.StrictParamDupe) {
            token = strict ? options.stricted : options.firstRestricted;
            throwUnexpectedToken(token, options.message);
        }

        if (defaultCount === 0) {
            defaults = [];
        }

        return {
            params: params,
            defaults: defaults,
            stricted: options.stricted,
            firstRestricted: options.firstRestricted,
            message: options.message
        };
    }

    function parseArrowFunctionExpression(options, node) {
        var previousStrict, body;

        if (hasLineTerminator) {
            tolerateUnexpectedToken(lookahead);
        }
        expect('=>');
        previousStrict = strict;

        body = parseConciseBody();

        if (strict && options.firstRestricted) {
            throwUnexpectedToken(options.firstRestricted, options.message);
        }
        if (strict && options.stricted) {
            tolerateUnexpectedToken(options.stricted, options.message);
        }

        strict = previousStrict;

        return node.finishArrowFunctionExpression(options.params, options.defaults, body, body.type !== Syntax.BlockStatement);
    }

    // [ES6] 14.4 Yield expression

    function parseYieldExpression() {
        var argument, expr, delegate;

        expr = new Node();

        if (!state.allowYield) {
            tolerateUnexpectedToken(lookahead, Messages.IllegalYield);
        }

        expectKeyword('yield');

        if (!hasLineTerminator) {
            delegate = match('*');
            if (delegate) {
                lex();
                argument = parseExpression();
            } else {
                if (!match(';') && !match('}') && lookahead.type !== Token.EOF) {
                    argument = parseExpression();
                }
            }
        }

        return expr.finishYieldExpression(argument, delegate);
    }

    // 11.13 Assignment Operators

    function parseAssignmentExpression() {
        var token, expr, right, list, startToken;

        startToken = lookahead;
        token = lookahead;

        if (matchKeyword('yield')) {
            return parseYieldExpression();
        }

        expr = parseConditionalExpression();

        if (expr.type === PlaceHolders.ArrowParameterPlaceHolder || match('=>')) {
            isAssignmentTarget = isBindingElement = false;
            list = reinterpretAsCoverFormalsList(expr);

            if (list) {
                firstCoverInitializedNameError = null;
                return parseArrowFunctionExpression(list, new WrappingNode(startToken));
            }

            return expr;
        }

        if (matchAssign()) {
            if (!isAssignmentTarget) {
                tolerateError(Messages.InvalidLHSInAssignment);
            }

            // 11.13.1
            if (strict && expr.type === Syntax.Identifier && isRestrictedWord(expr.name)) {
                tolerateUnexpectedToken(token, Messages.StrictLHSAssignment);
            }

            if (!match('=')) {
                isAssignmentTarget = isBindingElement = false;
            } else {
                reinterpretExpressionAsPattern(expr);
            }

            token = lex();
            right = isolateCoverGrammar(parseAssignmentExpression);
            expr = new WrappingNode(startToken).finishAssignmentExpression(token.value, expr, right);
            firstCoverInitializedNameError = null;
        }

        return expr;
    }

    // 11.14 Comma Operator

    function parseExpression() {
        var expr, startToken = lookahead, expressions;

        expr = isolateCoverGrammar(parseAssignmentExpression);

        if (match(',')) {
            expressions = [expr];

            while (startIndex < length) {
                if (!match(',')) {
                    break;
                }
                lex();
                expressions.push(isolateCoverGrammar(parseAssignmentExpression));
            }

            expr = new WrappingNode(startToken).finishSequenceExpression(expressions);
        }

        return expr;
    }

    // 12.1 Block

    function parseStatementListItem() {
        if (lookahead.type === Token.Keyword) {
            switch (lookahead.value) {
            case 'export':
                if (sourceType !== 'module') {
                    tolerateUnexpectedToken(lookahead, Messages.IllegalExportDeclaration);
                }
                return parseExportDeclaration();
            case 'import':
                if (sourceType !== 'module') {
                    tolerateUnexpectedToken(lookahead, Messages.IllegalImportDeclaration);
                }
                return parseImportDeclaration();
            case 'const':
            case 'let':
                return parseLexicalDeclaration({inFor: false});
            case 'function':
                return parseFunctionDeclaration(new Node());
            case 'class':
                return parseClassDeclaration();
            }
        }

        return parseStatement();
    }

    function parseStatementList() {
        var list = [];
        while (startIndex < length) {
            if (match('}')) {
                break;
            }
            list.push(parseStatementListItem());
        }

        return list;
    }

    function parseBlock() {
        var block, node = new Node();

        expect('{');

        block = parseStatementList();

        expect('}');

        return node.finishBlockStatement(block);
    }

    // 12.2 Variable Statement

    function parseVariableIdentifier() {
        var token, node = new Node();

        token = lex();

        if (token.type !== Token.Identifier) {
            if (strict && token.type === Token.Keyword && isStrictModeReservedWord(token.value)) {
                tolerateUnexpectedToken(token, Messages.StrictReservedWord);
            } else {
                throwUnexpectedToken(token);
            }
        }

        return node.finishIdentifier(token.value);
    }

    function parseVariableDeclaration() {
        var init = null, id, node = new Node();

        id = parsePattern();

        // 12.2.1
        if (strict && isRestrictedWord(id.name)) {
            tolerateError(Messages.StrictVarName);
        }

        if (match('=')) {
            lex();
            init = isolateCoverGrammar(parseAssignmentExpression);
        } else if (id.type !== Syntax.Identifier) {
            expect('=');
        }

        return node.finishVariableDeclarator(id, init);
    }

    function parseVariableDeclarationList() {
        var list = [];

        do {
            list.push(parseVariableDeclaration());
            if (!match(',')) {
                break;
            }
            lex();
        } while (startIndex < length);

        return list;
    }

    function parseVariableStatement(node) {
        var declarations;

        expectKeyword('var');

        declarations = parseVariableDeclarationList();

        consumeSemicolon();

        return node.finishVariableDeclaration(declarations);
    }

    function parseLexicalBinding(kind, options) {
        var init = null, id, node = new Node();

        id = parsePattern();

        // 12.2.1
        if (strict && id.type === Syntax.Identifier && isRestrictedWord(id.name)) {
            tolerateError(Messages.StrictVarName);
        }

        if (kind === 'const') {
            if (!matchKeyword('in') && !matchContextualKeyword('of')) {
                expect('=');
                init = isolateCoverGrammar(parseAssignmentExpression);
            }
        } else if ((!options.inFor && id.type !== Syntax.Identifier) || match('=')) {
            expect('=');
            init = isolateCoverGrammar(parseAssignmentExpression);
        }

        return node.finishVariableDeclarator(id, init);
    }

    function parseBindingList(kind, options) {
        var list = [];

        do {
            list.push(parseLexicalBinding(kind, options));
            if (!match(',')) {
                break;
            }
            lex();
        } while (startIndex < length);

        return list;
    }

    function parseLexicalDeclaration(options) {
        var kind, declarations, node = new Node();

        kind = lex().value;
        assert(kind === 'let' || kind === 'const', 'Lexical declaration must be either let or const');

        declarations = parseBindingList(kind, options);

        consumeSemicolon();

        return node.finishLexicalDeclaration(declarations, kind);
    }

    function parseRestElement() {
        var param, node = new Node();

        lex();

        if (match('{')) {
            throwError(Messages.ObjectPatternAsRestParameter);
        }

        param = parseVariableIdentifier();

        if (match('=')) {
            throwError(Messages.DefaultRestParameter);
        }

        if (!match(')')) {
            throwError(Messages.ParameterAfterRestParameter);
        }

        return node.finishRestElement(param);
    }

    // 12.3 Empty Statement

    function parseEmptyStatement(node) {
        expect(';');
        return node.finishEmptyStatement();
    }

    // 12.4 Expression Statement

    function parseExpressionStatement(node) {
        var expr = parseExpression();
        consumeSemicolon();
        return node.finishExpressionStatement(expr);
    }

    // 12.5 If statement

    function parseIfStatement(node) {
        var test, consequent, alternate;

        expectKeyword('if');

        expect('(');

        test = parseExpression();

        expect(')');

        consequent = parseStatement();

        if (matchKeyword('else')) {
            lex();
            alternate = parseStatement();
        } else {
            alternate = null;
        }

        return node.finishIfStatement(test, consequent, alternate);
    }

    // 12.6 Iteration Statements

    function parseDoWhileStatement(node) {
        var body, test, oldInIteration;

        expectKeyword('do');

        oldInIteration = state.inIteration;
        state.inIteration = true;

        body = parseStatement();

        state.inIteration = oldInIteration;

        expectKeyword('while');

        expect('(');

        test = parseExpression();

        expect(')');

        if (match(';')) {
            lex();
        }

        return node.finishDoWhileStatement(body, test);
    }

    function parseWhileStatement(node) {
        var test, body, oldInIteration;

        expectKeyword('while');

        expect('(');

        test = parseExpression();

        expect(')');

        oldInIteration = state.inIteration;
        state.inIteration = true;

        body = parseStatement();

        state.inIteration = oldInIteration;

        return node.finishWhileStatement(test, body);
    }

    function parseForStatement(node) {
        var init, forIn, initSeq, initStartToken, test, update, left, right, kind, declarations,
            body, oldInIteration, previousAllowIn = state.allowIn;

        init = test = update = null;
        forIn = true;

        expectKeyword('for');

        expect('(');

        if (match(';')) {
            lex();
        } else {
            if (matchKeyword('var')) {
                init = new Node();
                lex();

                state.allowIn = false;
                init = init.finishVariableDeclaration(parseVariableDeclarationList());
                state.allowIn = previousAllowIn;

                if (init.declarations.length === 1 && matchKeyword('in')) {
                    lex();
                    left = init;
                    right = parseExpression();
                    init = null;
                } else if (init.declarations.length === 1 && init.declarations[0].init === null && matchContextualKeyword('of')) {
                    lex();
                    left = init;
                    right = parseAssignmentExpression();
                    init = null;
                    forIn = false;
                } else {
                    expect(';');
                }
            } else if (matchKeyword('const') || matchKeyword('let')) {
                init = new Node();
                kind = lex().value;

                state.allowIn = false;
                declarations = parseBindingList(kind, {inFor: true});
                state.allowIn = previousAllowIn;

                if (declarations.length === 1 && declarations[0].init === null && matchKeyword('in')) {
                    init = init.finishLexicalDeclaration(declarations, kind);
                    lex();
                    left = init;
                    right = parseExpression();
                    init = null;
                } else if (declarations.length === 1 && declarations[0].init === null && matchContextualKeyword('of')) {
                    init = init.finishLexicalDeclaration(declarations, kind);
                    lex();
                    left = init;
                    right = parseAssignmentExpression();
                    init = null;
                    forIn = false;
                } else {
                    consumeSemicolon();
                    init = init.finishLexicalDeclaration(declarations, kind);
                }
            } else {
                initStartToken = lookahead;
                state.allowIn = false;
                init = inheritCoverGrammar(parseAssignmentExpression);
                state.allowIn = previousAllowIn;

                if (matchKeyword('in')) {
                    if (!isAssignmentTarget) {
                        tolerateError(Messages.InvalidLHSInForIn);
                    }

                    lex();
                    reinterpretExpressionAsPattern(init);
                    left = init;
                    right = parseExpression();
                    init = null;
                } else if (matchContextualKeyword('of')) {
                    if (!isAssignmentTarget) {
                        tolerateError(Messages.InvalidLHSInForLoop);
                    }

                    lex();
                    reinterpretExpressionAsPattern(init);
                    left = init;
                    right = parseAssignmentExpression();
                    init = null;
                    forIn = false;
                } else {
                    if (match(',')) {
                        initSeq = [init];
                        while (match(',')) {
                            lex();
                            initSeq.push(isolateCoverGrammar(parseAssignmentExpression));
                        }
                        init = new WrappingNode(initStartToken).finishSequenceExpression(initSeq);
                    }
                    expect(';');
                }
            }
        }

        if (typeof left === 'undefined') {

            if (!match(';')) {
                test = parseExpression();
            }
            expect(';');

            if (!match(')')) {
                update = parseExpression();
            }
        }

        expect(')');

        oldInIteration = state.inIteration;
        state.inIteration = true;

        body = isolateCoverGrammar(parseStatement);

        state.inIteration = oldInIteration;

        return (typeof left === 'undefined') ?
                node.finishForStatement(init, test, update, body) :
                forIn ? node.finishForInStatement(left, right, body) :
                    node.finishForOfStatement(left, right, body);
    }

    // 12.7 The continue statement

    function parseContinueStatement(node) {
        var label = null, key;

        expectKeyword('continue');

        // Optimize the most common form: 'continue;'.
        if (source.charCodeAt(startIndex) === 0x3B) {
            lex();

            if (!state.inIteration) {
                throwError(Messages.IllegalContinue);
            }

            return node.finishContinueStatement(null);
        }

        if (hasLineTerminator) {
            if (!state.inIteration) {
                throwError(Messages.IllegalContinue);
            }

            return node.finishContinueStatement(null);
        }

        if (lookahead.type === Token.Identifier) {
            label = parseVariableIdentifier();

            key = '$' + label.name;
            if (!Object.prototype.hasOwnProperty.call(state.labelSet, key)) {
                throwError(Messages.UnknownLabel, label.name);
            }
        }

        consumeSemicolon();

        if (label === null && !state.inIteration) {
            throwError(Messages.IllegalContinue);
        }

        return node.finishContinueStatement(label);
    }

    // 12.8 The break statement

    function parseBreakStatement(node) {
        var label = null, key;

        expectKeyword('break');

        // Catch the very common case first: immediately a semicolon (U+003B).
        if (source.charCodeAt(lastIndex) === 0x3B) {
            lex();

            if (!(state.inIteration || state.inSwitch)) {
                throwError(Messages.IllegalBreak);
            }

            return node.finishBreakStatement(null);
        }

        if (hasLineTerminator) {
            if (!(state.inIteration || state.inSwitch)) {
                throwError(Messages.IllegalBreak);
            }

            return node.finishBreakStatement(null);
        }

        if (lookahead.type === Token.Identifier) {
            label = parseVariableIdentifier();

            key = '$' + label.name;
            if (!Object.prototype.hasOwnProperty.call(state.labelSet, key)) {
                throwError(Messages.UnknownLabel, label.name);
            }
        }

        consumeSemicolon();

        if (label === null && !(state.inIteration || state.inSwitch)) {
            throwError(Messages.IllegalBreak);
        }

        return node.finishBreakStatement(label);
    }

    // 12.9 The return statement

    function parseReturnStatement(node) {
        var argument = null;

        expectKeyword('return');

        if (!state.inFunctionBody) {
            tolerateError(Messages.IllegalReturn);
        }

        // 'return' followed by a space and an identifier is very common.
        if (source.charCodeAt(lastIndex) === 0x20) {
            if (isIdentifierStart(source.charCodeAt(lastIndex + 1))) {
                argument = parseExpression();
                consumeSemicolon();
                return node.finishReturnStatement(argument);
            }
        }

        if (hasLineTerminator) {
            // HACK
            return node.finishReturnStatement(null);
        }

        if (!match(';')) {
            if (!match('}') && lookahead.type !== Token.EOF) {
                argument = parseExpression();
            }
        }

        consumeSemicolon();

        return node.finishReturnStatement(argument);
    }

    // 12.10 The with statement

    function parseWithStatement(node) {
        var object, body;

        if (strict) {
            tolerateError(Messages.StrictModeWith);
        }

        expectKeyword('with');

        expect('(');

        object = parseExpression();

        expect(')');

        body = parseStatement();

        return node.finishWithStatement(object, body);
    }

    // 12.10 The swith statement

    function parseSwitchCase() {
        var test, consequent = [], statement, node = new Node();

        if (matchKeyword('default')) {
            lex();
            test = null;
        } else {
            expectKeyword('case');
            test = parseExpression();
        }
        expect(':');

        while (startIndex < length) {
            if (match('}') || matchKeyword('default') || matchKeyword('case')) {
                break;
            }
            statement = parseStatementListItem();
            consequent.push(statement);
        }

        return node.finishSwitchCase(test, consequent);
    }

    function parseSwitchStatement(node) {
        var discriminant, cases, clause, oldInSwitch, defaultFound;

        expectKeyword('switch');

        expect('(');

        discriminant = parseExpression();

        expect(')');

        expect('{');

        cases = [];

        if (match('}')) {
            lex();
            return node.finishSwitchStatement(discriminant, cases);
        }

        oldInSwitch = state.inSwitch;
        state.inSwitch = true;
        defaultFound = false;

        while (startIndex < length) {
            if (match('}')) {
                break;
            }
            clause = parseSwitchCase();
            if (clause.test === null) {
                if (defaultFound) {
                    throwError(Messages.MultipleDefaultsInSwitch);
                }
                defaultFound = true;
            }
            cases.push(clause);
        }

        state.inSwitch = oldInSwitch;

        expect('}');

        return node.finishSwitchStatement(discriminant, cases);
    }

    // 12.13 The throw statement

    function parseThrowStatement(node) {
        var argument;

        expectKeyword('throw');

        if (hasLineTerminator) {
            throwError(Messages.NewlineAfterThrow);
        }

        argument = parseExpression();

        consumeSemicolon();

        return node.finishThrowStatement(argument);
    }

    // 12.14 The try statement

    function parseCatchClause() {
        var param, body, node = new Node();

        expectKeyword('catch');

        expect('(');
        if (match(')')) {
            throwUnexpectedToken(lookahead);
        }

        param = parsePattern();

        // 12.14.1
        if (strict && isRestrictedWord(param.name)) {
            tolerateError(Messages.StrictCatchVariable);
        }

        expect(')');
        body = parseBlock();
        return node.finishCatchClause(param, body);
    }

    function parseTryStatement(node) {
        var block, handler = null, finalizer = null;

        expectKeyword('try');

        block = parseBlock();

        if (matchKeyword('catch')) {
            handler = parseCatchClause();
        }

        if (matchKeyword('finally')) {
            lex();
            finalizer = parseBlock();
        }

        if (!handler && !finalizer) {
            throwError(Messages.NoCatchOrFinally);
        }

        return node.finishTryStatement(block, handler, finalizer);
    }

    // 12.15 The debugger statement

    function parseDebuggerStatement(node) {
        expectKeyword('debugger');

        consumeSemicolon();

        return node.finishDebuggerStatement();
    }

    // 12 Statements

    function parseStatement() {
        var type = lookahead.type,
            expr,
            labeledBody,
            key,
            node;

        if (type === Token.EOF) {
            throwUnexpectedToken(lookahead);
        }

        if (type === Token.Punctuator && lookahead.value === '{') {
            return parseBlock();
        }
        isAssignmentTarget = isBindingElement = true;
        node = new Node();

        if (type === Token.Punctuator) {
            switch (lookahead.value) {
            case ';':
                return parseEmptyStatement(node);
            case '(':
                return parseExpressionStatement(node);
            default:
                break;
            }
        } else if (type === Token.Keyword) {
            switch (lookahead.value) {
            case 'break':
                return parseBreakStatement(node);
            case 'continue':
                return parseContinueStatement(node);
            case 'debugger':
                return parseDebuggerStatement(node);
            case 'do':
                return parseDoWhileStatement(node);
            case 'for':
                return parseForStatement(node);
            case 'function':
                return parseFunctionDeclaration(node);
            case 'if':
                return parseIfStatement(node);
            case 'return':
                return parseReturnStatement(node);
            case 'switch':
                return parseSwitchStatement(node);
            case 'throw':
                return parseThrowStatement(node);
            case 'try':
                return parseTryStatement(node);
            case 'var':
                return parseVariableStatement(node);
            case 'while':
                return parseWhileStatement(node);
            case 'with':
                return parseWithStatement(node);
            default:
                break;
            }
        }

        expr = parseExpression();

        // 12.12 Labelled Statements
        if ((expr.type === Syntax.Identifier) && match(':')) {
            lex();

            key = '$' + expr.name;
            if (Object.prototype.hasOwnProperty.call(state.labelSet, key)) {
                throwError(Messages.Redeclaration, 'Label', expr.name);
            }

            state.labelSet[key] = true;
            labeledBody = parseStatement();
            delete state.labelSet[key];
            return node.finishLabeledStatement(expr, labeledBody);
        }

        consumeSemicolon();

        return node.finishExpressionStatement(expr);
    }

    // 13 Function Definition

    function parseFunctionSourceElements() {
        var statement, body = [], token, directive, firstRestricted,
            oldLabelSet, oldInIteration, oldInSwitch, oldInFunctionBody, oldParenthesisCount,
            node = new Node();

        expect('{');

        while (startIndex < length) {
            if (lookahead.type !== Token.StringLiteral) {
                break;
            }
            token = lookahead;

            statement = parseStatementListItem();
            body.push(statement);
            if (statement.expression.type !== Syntax.Literal) {
                // this is not directive
                break;
            }
            directive = source.slice(token.start + 1, token.end - 1);
            if (directive === 'use strict') {
                strict = true;
                if (firstRestricted) {
                    tolerateUnexpectedToken(firstRestricted, Messages.StrictOctalLiteral);
                }
            } else {
                if (!firstRestricted && token.octal) {
                    firstRestricted = token;
                }
            }
        }

        oldLabelSet = state.labelSet;
        oldInIteration = state.inIteration;
        oldInSwitch = state.inSwitch;
        oldInFunctionBody = state.inFunctionBody;
        oldParenthesisCount = state.parenthesizedCount;

        state.labelSet = {};
        state.inIteration = false;
        state.inSwitch = false;
        state.inFunctionBody = true;
        state.parenthesizedCount = 0;

        while (startIndex < length) {
            if (match('}')) {
                break;
            }
            body.push(parseStatementListItem());
        }

        expect('}');

        state.labelSet = oldLabelSet;
        state.inIteration = oldInIteration;
        state.inSwitch = oldInSwitch;
        state.inFunctionBody = oldInFunctionBody;
        state.parenthesizedCount = oldParenthesisCount;

        return node.finishBlockStatement(body);
    }

    function validateParam(options, param, name) {
        var key = '$' + name;
        if (strict) {
            if (isRestrictedWord(name)) {
                options.stricted = param;
                options.message = Messages.StrictParamName;
            }
            if (Object.prototype.hasOwnProperty.call(options.paramSet, key)) {
                options.stricted = param;
                options.message = Messages.StrictParamDupe;
            }
        } else if (!options.firstRestricted) {
            if (isRestrictedWord(name)) {
                options.firstRestricted = param;
                options.message = Messages.StrictParamName;
            } else if (isStrictModeReservedWord(name)) {
                options.firstRestricted = param;
                options.message = Messages.StrictReservedWord;
            } else if (Object.prototype.hasOwnProperty.call(options.paramSet, key)) {
                options.firstRestricted = param;
                options.message = Messages.StrictParamDupe;
            }
        }
        options.paramSet[key] = true;
    }

    function parseParam(options) {
        var token, param, def;

        token = lookahead;
        if (token.value === '...') {
            param = parseRestElement();
            validateParam(options, param.argument, param.argument.name);
            options.params.push(param);
            options.defaults.push(null);
            return false;
        }

        param = parsePatternWithDefault();
        validateParam(options, token, token.value);

        if (param.type === Syntax.AssignmentPattern) {
            def = param.right;
            param = param.left;
            ++options.defaultCount;
        }

        options.params.push(param);
        options.defaults.push(def);

        return !match(')');
    }

    function parseParams(firstRestricted) {
        var options;

        options = {
            params: [],
            defaultCount: 0,
            defaults: [],
            firstRestricted: firstRestricted
        };

        expect('(');

        if (!match(')')) {
            options.paramSet = {};
            while (startIndex < length) {
                if (!parseParam(options)) {
                    break;
                }
                expect(',');
            }
        }

        expect(')');

        if (options.defaultCount === 0) {
            options.defaults = [];
        }

        return {
            params: options.params,
            defaults: options.defaults,
            stricted: options.stricted,
            firstRestricted: options.firstRestricted,
            message: options.message
        };
    }

    function parseFunctionDeclaration(node, identifierIsOptional) {
        var id = null, params = [], defaults = [], body, token, stricted, tmp, firstRestricted, message, previousStrict,
            isGenerator, previousAllowYield;

        expectKeyword('function');

        isGenerator = match('*');
        if (isGenerator) {
            lex();
        }

        if (!identifierIsOptional || !match('(')) {
            token = lookahead;
            id = parseVariableIdentifier();
            if (strict) {
                if (isRestrictedWord(token.value)) {
                    tolerateUnexpectedToken(token, Messages.StrictFunctionName);
                }
            } else {
                if (isRestrictedWord(token.value)) {
                    firstRestricted = token;
                    message = Messages.StrictFunctionName;
                } else if (isStrictModeReservedWord(token.value)) {
                    firstRestricted = token;
                    message = Messages.StrictReservedWord;
                }
            }
        }

        previousAllowYield = state.allowYield;
        state.allowYield = false;
        tmp = parseParams(firstRestricted);
        state.allowYield = previousAllowYield;

        params = tmp.params;
        defaults = tmp.defaults;
        stricted = tmp.stricted;
        firstRestricted = tmp.firstRestricted;
        if (tmp.message) {
            message = tmp.message;
        }

        previousAllowYield = state.allowYield;
        previousStrict = strict;
        state.allowYield = isGenerator;
        body = parseFunctionSourceElements();
        if (strict && firstRestricted) {
            throwUnexpectedToken(firstRestricted, message);
        }
        if (strict && stricted) {
            tolerateUnexpectedToken(stricted, message);
        }
        strict = previousStrict;
        state.allowYield = previousAllowYield;

        return node.finishFunctionDeclaration(id, params, defaults, body, isGenerator);
    }

    function parseFunctionExpression() {
        var token, id = null, stricted, firstRestricted, message, tmp,
            params = [], defaults = [], body, previousStrict, node = new Node(),
            isGenerator, previousAllowYield;

        expectKeyword('function');

        isGenerator = match('*');
        if (isGenerator) {
            lex();
        }

        if (!match('(')) {
            token = lookahead;
            id = parseVariableIdentifier();
            if (strict) {
                if (isRestrictedWord(token.value)) {
                    tolerateUnexpectedToken(token, Messages.StrictFunctionName);
                }
            } else {
                if (isRestrictedWord(token.value)) {
                    firstRestricted = token;
                    message = Messages.StrictFunctionName;
                } else if (isStrictModeReservedWord(token.value)) {
                    firstRestricted = token;
                    message = Messages.StrictReservedWord;
                }
            }
        }

        previousAllowYield = state.allowYield;
        state.allowYield = false;
        tmp = parseParams(firstRestricted);
        state.allowYield = previousAllowYield;

        params = tmp.params;
        defaults = tmp.defaults;
        stricted = tmp.stricted;
        firstRestricted = tmp.firstRestricted;
        if (tmp.message) {
            message = tmp.message;
        }

        previousStrict = strict;
        previousAllowYield = state.allowYield;
        state.allowYield = isGenerator;
        body = parseFunctionSourceElements();

        if (strict && firstRestricted) {
            throwUnexpectedToken(firstRestricted, message);
        }
        if (strict && stricted) {
            tolerateUnexpectedToken(stricted, message);
        }
        strict = previousStrict;
        state.allowYield = previousAllowYield;

        return node.finishFunctionExpression(id, params, defaults, body, isGenerator);
    }


    function parseClassBody() {
        var classBody, token, isStatic, hasConstructor = false, body, method, computed, key;

        classBody = new Node();

        expect('{');
        body = [];
        while (!match('}')) {
            if (match(';')) {
                lex();
            } else {
                method = new Node();
                token = lookahead;
                isStatic = false;
                computed = match('[');
                if (match('*')) {
                    lex();
                } else {
                    key = parseObjectPropertyKey();
                    if (key.name === 'static' && (lookaheadPropertyName() || match('*'))) {
                        token = lookahead;
                        isStatic = true;
                        computed = match('[');
                        if (match('*')) {
                            lex();
                        } else {
                            key = parseObjectPropertyKey();
                        }
                    }
                }
                method = tryParseMethodDefinition(token, key, computed, method);
                if (method) {
                    method['static'] = isStatic;
                    if (method.kind === 'init') {
                        method.kind = 'method';
                    }
                    if (!isStatic) {
                        if (!method.computed && (method.key.name || method.key.value.toString()) === 'constructor') {
                            if (method.kind !== 'method' || !method.method || method.value.generator) {
                                throwUnexpectedToken(token, Messages.ConstructorSpecialMethod);
                            }
                            if (hasConstructor) {
                                throwUnexpectedToken(token, Messages.DuplicateConstructor);
                            } else {
                                hasConstructor = true;
                            }
                            method.kind = 'constructor';
                        }
                    } else {
                        if (!method.computed && (method.key.name || method.key.value.toString()) === 'prototype') {
                            throwUnexpectedToken(token, Messages.StaticPrototype);
                        }
                    }
                    method.type = Syntax.MethodDefinition;
                    delete method.method;
                    delete method.shorthand;
                    body.push(method);
                } else {
                    throwUnexpectedToken(lookahead);
                }
            }
        }
        lex();
        return classBody.finishClassBody(body);
    }

    function parseClassDeclaration(identifierIsOptional) {
        var id = null, superClass = null, classNode = new Node(), classBody, previousStrict = strict;
        strict = true;

        expectKeyword('class');

        if (!identifierIsOptional || lookahead.type === Token.Identifier) {
            id = parseVariableIdentifier();
        }

        if (matchKeyword('extends')) {
            lex();
            superClass = isolateCoverGrammar(parseLeftHandSideExpressionAllowCall);
        }
        classBody = parseClassBody();
        strict = previousStrict;

        return classNode.finishClassDeclaration(id, superClass, classBody);
    }

    function parseClassExpression() {
        var id = null, superClass = null, classNode = new Node(), classBody, previousStrict = strict;
        strict = true;

        expectKeyword('class');

        if (lookahead.type === Token.Identifier) {
            id = parseVariableIdentifier();
        }

        if (matchKeyword('extends')) {
            lex();
            superClass = isolateCoverGrammar(parseLeftHandSideExpressionAllowCall);
        }
        classBody = parseClassBody();
        strict = previousStrict;

        return classNode.finishClassExpression(id, superClass, classBody);
    }

    // Modules grammar from:
    // people.mozilla.org/~jorendorff/es6-draft.html

    function parseModuleSpecifier() {
        var node = new Node();

        if (lookahead.type !== Token.StringLiteral) {
            throwError(Messages.InvalidModuleSpecifier);
        }
        return node.finishLiteral(lex());
    }

    function parseExportSpecifier() {
        var exported, local, node = new Node(), def;
        if (matchKeyword('default')) {
            // export {default} from 'something';
            def = new Node();
            lex();
            local = def.finishIdentifier('default');
        } else {
            local = parseVariableIdentifier();
        }
        if (matchContextualKeyword('as')) {
            lex();
            exported = parseNonComputedProperty();
        }
        return node.finishExportSpecifier(local, exported);
    }

    function parseExportNamedDeclaration(node) {
        var declaration = null,
            isExportFromIdentifier,
            src = null, specifiers = [];

        // non-default export
        if (lookahead.type === Token.Keyword) {
            // covers:
            // export var f = 1;
            switch (lookahead.value) {
                case 'let':
                case 'const':
                case 'var':
                case 'class':
                case 'function':
                    declaration = parseStatementListItem();
                    return node.finishExportNamedDeclaration(declaration, specifiers, null);
            }
        }

        expect('{');
        if (!match('}')) {
            do {
                isExportFromIdentifier = isExportFromIdentifier || matchKeyword('default');
                specifiers.push(parseExportSpecifier());
            } while (match(',') && lex());
        }
        expect('}');

        if (matchContextualKeyword('from')) {
            // covering:
            // export {default} from 'foo';
            // export {foo} from 'foo';
            lex();
            src = parseModuleSpecifier();
            consumeSemicolon();
        } else if (isExportFromIdentifier) {
            // covering:
            // export {default}; // missing fromClause
            throwError(lookahead.value ?
                    Messages.UnexpectedToken : Messages.MissingFromClause, lookahead.value);
        } else {
            // cover
            // export {foo};
            consumeSemicolon();
        }
        return node.finishExportNamedDeclaration(declaration, specifiers, src);
    }

    function parseExportDefaultDeclaration(node) {
        var declaration = null,
            expression = null;

        // covers:
        // export default ...
        expectKeyword('default');

        if (matchKeyword('function')) {
            // covers:
            // export default function foo () {}
            // export default function () {}
            declaration = parseFunctionDeclaration(new Node(), true);
            return node.finishExportDefaultDeclaration(declaration);
        }
        if (matchKeyword('class')) {
            declaration = parseClassDeclaration(true);
            return node.finishExportDefaultDeclaration(declaration);
        }

        if (matchContextualKeyword('from')) {
            throwError(Messages.UnexpectedToken, lookahead.value);
        }

        // covers:
        // export default {};
        // export default [];
        // export default (1 + 2);
        if (match('{')) {
            expression = parseObjectInitialiser();
        } else if (match('[')) {
            expression = parseArrayInitialiser();
        } else {
            expression = parseAssignmentExpression();
        }
        consumeSemicolon();
        return node.finishExportDefaultDeclaration(expression);
    }

    function parseExportAllDeclaration(node) {
        var src;

        // covers:
        // export * from 'foo';
        expect('*');
        if (!matchContextualKeyword('from')) {
            throwError(lookahead.value ?
                    Messages.UnexpectedToken : Messages.MissingFromClause, lookahead.value);
        }
        lex();
        src = parseModuleSpecifier();
        consumeSemicolon();

        return node.finishExportAllDeclaration(src);
    }

    function parseExportDeclaration() {
        var node = new Node();
        if (state.inFunctionBody) {
            throwError(Messages.IllegalExportDeclaration);
        }

        expectKeyword('export');

        if (matchKeyword('default')) {
            return parseExportDefaultDeclaration(node);
        }
        if (match('*')) {
            return parseExportAllDeclaration(node);
        }
        return parseExportNamedDeclaration(node);
    }

    function parseImportSpecifier() {
        // import {<foo as bar>} ...;
        var local, imported, node = new Node();

        imported = parseNonComputedProperty();
        if (matchContextualKeyword('as')) {
            lex();
            local = parseVariableIdentifier();
        }

        return node.finishImportSpecifier(local, imported);
    }

    function parseNamedImports() {
        var specifiers = [];
        // {foo, bar as bas}
        expect('{');
        if (!match('}')) {
            do {
                specifiers.push(parseImportSpecifier());
            } while (match(',') && lex());
        }
        expect('}');
        return specifiers;
    }

    function parseImportDefaultSpecifier() {
        // import <foo> ...;
        var local, node = new Node();

        local = parseNonComputedProperty();

        return node.finishImportDefaultSpecifier(local);
    }

    function parseImportNamespaceSpecifier() {
        // import <* as foo> ...;
        var local, node = new Node();

        expect('*');
        if (!matchContextualKeyword('as')) {
            throwError(Messages.NoAsAfterImportNamespace);
        }
        lex();
        local = parseNonComputedProperty();

        return node.finishImportNamespaceSpecifier(local);
    }

    function parseImportDeclaration() {
        var specifiers, src, node = new Node();

        if (state.inFunctionBody) {
            throwError(Messages.IllegalImportDeclaration);
        }

        expectKeyword('import');
        specifiers = [];

        if (lookahead.type === Token.StringLiteral) {
            // covers:
            // import 'foo';
            src = parseModuleSpecifier();
            consumeSemicolon();
            return node.finishImportDeclaration(specifiers, src);
        }

        if (!matchKeyword('default') && isIdentifierName(lookahead)) {
            // covers:
            // import foo
            // import foo, ...
            specifiers.push(parseImportDefaultSpecifier());
            if (match(',')) {
                lex();
            }
        }
        if (match('*')) {
            // covers:
            // import foo, * as foo
            // import * as foo
            specifiers.push(parseImportNamespaceSpecifier());
        } else if (match('{')) {
            // covers:
            // import foo, {bar}
            // import {bar}
            specifiers = specifiers.concat(parseNamedImports());
        }

        if (!matchContextualKeyword('from')) {
            throwError(lookahead.value ?
                    Messages.UnexpectedToken : Messages.MissingFromClause, lookahead.value);
        }
        lex();
        src = parseModuleSpecifier();
        consumeSemicolon();

        return node.finishImportDeclaration(specifiers, src);
    }

    // 14 Program

    function parseScriptBody() {
        var statement, body = [], token, directive, firstRestricted;

        while (startIndex < length) {
            token = lookahead;
            if (token.type !== Token.StringLiteral) {
                break;
            }

            statement = parseStatementListItem();
            body.push(statement);
            if (statement.expression.type !== Syntax.Literal) {
                // this is not directive
                break;
            }
            directive = source.slice(token.start + 1, token.end - 1);
            if (directive === 'use strict') {
                strict = true;
                if (firstRestricted) {
                    tolerateUnexpectedToken(firstRestricted, Messages.StrictOctalLiteral);
                }
            } else {
                if (!firstRestricted && token.octal) {
                    firstRestricted = token;
                }
            }
        }

        while (startIndex < length) {
            statement = parseStatementListItem();
            /* istanbul ignore if */
            if (typeof statement === 'undefined') {
                break;
            }
            body.push(statement);
        }
        return body;
    }

    function parseProgram() {
        var body, node;

        peek();
        node = new Node();

        body = parseScriptBody();
        return node.finishProgram(body);
    }

    function filterTokenLocation() {
        var i, entry, token, tokens = [];

        for (i = 0; i < extra.tokens.length; ++i) {
            entry = extra.tokens[i];
            token = {
                type: entry.type,
                value: entry.value
            };
            if (entry.regex) {
                token.regex = {
                    pattern: entry.regex.pattern,
                    flags: entry.regex.flags
                };
            }
            if (extra.range) {
                token.range = entry.range;
            }
            if (extra.loc) {
                token.loc = entry.loc;
            }
            tokens.push(token);
        }

        extra.tokens = tokens;
    }

    function tokenize(code, options) {
        var toString,
            tokens;

        toString = String;
        if (typeof code !== 'string' && !(code instanceof String)) {
            code = toString(code);
        }

        source = code;
        index = 0;
        lineNumber = (source.length > 0) ? 1 : 0;
        lineStart = 0;
        startIndex = index;
        startLineNumber = lineNumber;
        startLineStart = lineStart;
        length = source.length;
        lookahead = null;
        state = {
            allowIn: true,
            allowYield: false,
            labelSet: {},
            inFunctionBody: false,
            inIteration: false,
            inSwitch: false,
            lastCommentStart: -1,
            curlyStack: []
        };

        extra = {};

        // Options matching.
        options = options || {};

        // Of course we collect tokens here.
        options.tokens = true;
        extra.tokens = [];
        extra.tokenize = true;
        // The following two fields are necessary to compute the Regex tokens.
        extra.openParenToken = -1;
        extra.openCurlyToken = -1;

        extra.range = (typeof options.range === 'boolean') && options.range;
        extra.loc = (typeof options.loc === 'boolean') && options.loc;

        if (typeof options.comment === 'boolean' && options.comment) {
            extra.comments = [];
        }
        if (typeof options.tolerant === 'boolean' && options.tolerant) {
            extra.errors = [];
        }

        try {
            peek();
            if (lookahead.type === Token.EOF) {
                return extra.tokens;
            }

            lex();
            while (lookahead.type !== Token.EOF) {
                try {
                    lex();
                } catch (lexError) {
                    if (extra.errors) {
                        recordError(lexError);
                        // We have to break on the first error
                        // to avoid infinite loops.
                        break;
                    } else {
                        throw lexError;
                    }
                }
            }

            filterTokenLocation();
            tokens = extra.tokens;
            if (typeof extra.comments !== 'undefined') {
                tokens.comments = extra.comments;
            }
            if (typeof extra.errors !== 'undefined') {
                tokens.errors = extra.errors;
            }
        } catch (e) {
            throw e;
        } finally {
            extra = {};
        }
        return tokens;
    }

    function parse(code, options) {
        var program, toString;

        toString = String;
        if (typeof code !== 'string' && !(code instanceof String)) {
            code = toString(code);
        }

        source = code;
        index = 0;
        lineNumber = (source.length > 0) ? 1 : 0;
        lineStart = 0;
        startIndex = index;
        startLineNumber = lineNumber;
        startLineStart = lineStart;
        length = source.length;
        lookahead = null;
        state = {
            allowIn: true,
            allowYield: false,
            labelSet: {},
            inFunctionBody: false,
            inIteration: false,
            inSwitch: false,
            lastCommentStart: -1,
            curlyStack: []
        };
        sourceType = 'script';
        strict = false;

        extra = {};
        if (typeof options !== 'undefined') {
            extra.range = (typeof options.range === 'boolean') && options.range;
            extra.loc = (typeof options.loc === 'boolean') && options.loc;
            extra.attachComment = (typeof options.attachComment === 'boolean') && options.attachComment;

            if (extra.loc && options.source !== null && options.source !== undefined) {
                extra.source = toString(options.source);
            }

            if (typeof options.tokens === 'boolean' && options.tokens) {
                extra.tokens = [];
            }
            if (typeof options.comment === 'boolean' && options.comment) {
                extra.comments = [];
            }
            if (typeof options.tolerant === 'boolean' && options.tolerant) {
                extra.errors = [];
            }
            if (extra.attachComment) {
                extra.range = true;
                extra.comments = [];
                extra.bottomRightStack = [];
                extra.trailingComments = [];
                extra.leadingComments = [];
            }
            if (options.sourceType === 'module') {
                // very restrictive condition for now
                sourceType = options.sourceType;
                strict = true;
            }
        }

        try {
            program = parseProgram();
            if (typeof extra.comments !== 'undefined') {
                program.comments = extra.comments;
            }
            if (typeof extra.tokens !== 'undefined') {
                filterTokenLocation();
                program.tokens = extra.tokens;
            }
            if (typeof extra.errors !== 'undefined') {
                program.errors = extra.errors;
            }
        } catch (e) {
            throw e;
        } finally {
            extra = {};
        }

        return program;
    }

    // Sync with *.json manifests.
    exports.version = '2.4.0';

    exports.tokenize = tokenize;

    exports.parse = parse;

    // Deep copy.
    /* istanbul ignore next */
    exports.Syntax = (function () {
        var name, types = {};

        if (typeof Object.create === 'function') {
            types = Object.create(null);
        }

        for (name in Syntax) {
            if (Syntax.hasOwnProperty(name)) {
                types[name] = Syntax[name];
            }
        }

        if (typeof Object.freeze === 'function') {
            Object.freeze(types);
        }

        return types;
    }());

}));
/* vim: set sw=4 ts=4 et tw=80 : */