Esprima .NET (BSD license) is a .NET port of the esprima.org project. It is a standard-compliant ECMAScript parser (also popularly known as JavaScript).

• Full support for ECMAScript 2019 (ECMA-262 10th Edition)
• Sensible syntax tree format as standardized by ESTree project
• Experimental support for JSX, a syntax extension for React
• Optional tracking of syntax node location (index-based and line-column)
• Heavily tested

Esprima can be used to perform lexical analysis (tokenization) or syntactic analysis (parsing) of a JavaScript program.

A simple C# example:

var parser = new JavaScriptParser();
var program = parser.ParseScript("const answer = 42");

You can control the behavior of the parser by initializing and passing a ParserOptions to the parser's constructor. (For the available options, see the XML comments of the ParserOptions class.)

Instead of ParseScript, you may use ParseModule or ParseExpression to make the parser treat the input as an ES6 module or as a plain JavaScript expression respectively.

In case the input is syntactically correct, each of these methods returns the root node of the resulting abstract syntax tree (AST), which you can freely analyze or transform. The library provides the AstVisitor and AstRewriter visitor classes to help you with such tasks.

When the input contains a severe syntax error, a ParserException is thrown. By catching it you can get details about the error. There are syntax errors though which can be tolerated by the parser. Such errors are ignored by default. You can record them by setting ParserOptions.ErrorHandler to an instance of CollectingErrorHandler. Alternatively, you can set ParserOptions.Tolerant to false to make the parser throw exceptions also in the case of tolerable syntax errors.

The library is able to write the AST (except for comments) back to JavaScript code:

var code = program.ToJavaScriptString(format: true);

It is also possible to serialize the AST into a JSON representation:

var astJson = program.ToJsonString(indent: "    ");

Considering the example above this call will return the following JSON:

{
    "type": "Program",
    "body": [
        {
            "type": "VariableDeclaration",
            "declarations": [
                {
                    "type": "VariableDeclarator",
                    "id": {
                        "type": "Identifier",
                        "name": "answer"
                    },
                    "init": {
                        "type": "Literal",
                        "value": 42,
                        "raw": "42"
                    }
                }
            ],
            "kind": "const"
        }
    ],
    "sourceType": "script",
    "strict": false
}

Here is a list of common JavaScript libraries and the time it takes to parse them, compared to the time it took for the same script using the original Esprima in Chrome.