feat(eslint-plugin): [no-useless-default-assignment] add rule #11720
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| --- | ||
| description: 'Disallow default values that will never be used.' | ||
| --- | ||
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| import Tabs from '@theme/Tabs'; | ||
| import TabItem from '@theme/TabItem'; | ||
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| > 🛑 This file is source code, not the primary documentation location! 🛑 | ||
| > | ||
| > See **https://typescript-eslint.io/rules/no-useless-default-assignment** for documentation. | ||
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| [Default parameters](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/Default_parameters) and [default values](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Destructuring_assignment#default_value) are only used if the parameter or property is `undefined`. | ||
| That can happen when a value is missing, or when one is provided and set to `undefined`. | ||
| If a non-`undefined` value is guaranteed to be provided, then there is no need to define a default. | ||
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| ## Examples | ||
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| <Tabs> | ||
| <TabItem value="❌ Incorrect"> | ||
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| ```ts | ||
| function Bar({ foo = '' }: { foo: string }) { | ||
| return foo; | ||
| } | ||
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| const { foo = '' } = { foo: 'bar' }; | ||
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| const [foo = ''] = ['bar']; | ||
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| [1, 2, 3].map((a = 42) => a + 1); | ||
| ``` | ||
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| </TabItem> | ||
| <TabItem value="✅ Correct"> | ||
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| ```ts | ||
| function Bar({ foo = '' }: { foo?: string }) { | ||
| return foo; | ||
| } | ||
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| const { foo = '' } = { foo: undefined }; | ||
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| const [foo = ''] = [undefined]; | ||
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| [1, 2, 3, undefined].map((a = 42) => a + 1); | ||
| ``` | ||
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| </TabItem> | ||
| </Tabs> | ||
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| ## When Not To Use It | ||
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| If you use default values defensively against runtime values that bypass type checking, or for documentation purposes, you may want to disable this rule. |
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1,263 @@ | ||
| import type { TSESTree } from '@typescript-eslint/utils'; | ||
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| import { AST_NODE_TYPES } from '@typescript-eslint/utils'; | ||
| import * as tsutils from 'ts-api-utils'; | ||
| import * as ts from 'typescript'; | ||
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| import { | ||
| createRule, | ||
| getParserServices, | ||
| isFunction, | ||
| isTypeAnyType, | ||
| isTypeFlagSet, | ||
| isTypeUnknownType, | ||
| nullThrows, | ||
| NullThrowsReasons, | ||
| } from '../util'; | ||
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| type MessageId = 'uselessDefaultAssignment' | 'uselessUndefined'; | ||
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| export default createRule<[], MessageId>({ | ||
| name: 'no-useless-default-assignment', | ||
| meta: { | ||
| type: 'suggestion', | ||
| docs: { | ||
| description: 'Disallow default values that will never be used', | ||
| recommended: 'strict', | ||
| requiresTypeChecking: true, | ||
| }, | ||
| fixable: 'code', | ||
| messages: { | ||
| uselessDefaultAssignment: | ||
| 'Default value is useless because the {{ type }} is not optional.', | ||
| uselessUndefined: | ||
| 'Default value is useless because it is undefined. Optional {{ type }}s are already undefined by default.', | ||
| }, | ||
| schema: [], | ||
| }, | ||
| defaultOptions: [], | ||
| create(context) { | ||
| const services = getParserServices(context); | ||
| const checker = services.program.getTypeChecker(); | ||
| const compilerOptions = services.program.getCompilerOptions(); | ||
| const isNoUncheckedIndexedAccess = tsutils.isCompilerOptionEnabled( | ||
| compilerOptions, | ||
| 'noUncheckedIndexedAccess', | ||
| ); | ||
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| function canBeUndefined(type: ts.Type): boolean { | ||
| if (isTypeAnyType(type) || isTypeUnknownType(type)) { | ||
| return true; | ||
| } | ||
| return tsutils | ||
| .unionConstituents(type) | ||
| .some(part => isTypeFlagSet(part, ts.TypeFlags.Undefined)); | ||
| } | ||
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| function getPropertyType( | ||
| objectType: ts.Type, | ||
| propertyName: string, | ||
| ): ts.Type | null { | ||
| const symbol = objectType.getProperty(propertyName); | ||
| if (!symbol) { | ||
| if (isNoUncheckedIndexedAccess) { | ||
| return null; | ||
| } | ||
| return objectType.getStringIndexType() ?? null; | ||
| } | ||
| return checker.getTypeOfSymbol(symbol); | ||
| } | ||
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| function getArrayElementType( | ||
| arrayType: ts.Type, | ||
| elementIndex: number, | ||
| ): ts.Type | null { | ||
| if (checker.isTupleType(arrayType)) { | ||
| const tupleArgs = checker.getTypeArguments(arrayType); | ||
| if (elementIndex < tupleArgs.length) { | ||
| return tupleArgs[elementIndex]; | ||
| } | ||
| } | ||
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| if (isNoUncheckedIndexedAccess) { | ||
| return null; | ||
| } | ||
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| return arrayType.getNumberIndexType() ?? null; | ||
| } | ||
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| function checkAssignmentPattern(node: TSESTree.AssignmentPattern): void { | ||
| if ( | ||
| node.right.type === AST_NODE_TYPES.Identifier && | ||
| node.right.name === 'undefined' | ||
| ) { | ||
| const type = | ||
| node.parent.type === AST_NODE_TYPES.Property || | ||
| node.parent.type === AST_NODE_TYPES.ArrayPattern | ||
| ? 'property' | ||
| : 'parameter'; | ||
| reportUselessDefault(node, type, 'uselessUndefined'); | ||
| return; | ||
| } | ||
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| const parent = node.parent; | ||
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| if ( | ||
| parent.type === AST_NODE_TYPES.ArrowFunctionExpression || | ||
| parent.type === AST_NODE_TYPES.FunctionExpression | ||
| ) { | ||
| const paramIndex = parent.params.indexOf(node); | ||
| if (paramIndex !== -1) { | ||
| const tsFunc = services.esTreeNodeToTSNodeMap.get(parent); | ||
| if (ts.isFunctionLike(tsFunc)) { | ||
| const contextualType = checker.getContextualType( | ||
| tsFunc as ts.Expression, | ||
| ); | ||
| if (!contextualType) { | ||
| return; | ||
| } | ||
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| const signatures = contextualType.getCallSignatures(); | ||
| const params = signatures[0].getParameters(); | ||
| if (paramIndex < params.length) { | ||
| const paramSymbol = params[paramIndex]; | ||
| if ((paramSymbol.flags & ts.SymbolFlags.Optional) === 0) { | ||
| const paramType = checker.getTypeOfSymbol(paramSymbol); | ||
| if (!canBeUndefined(paramType)) { | ||
| reportUselessDefault( | ||
| node, | ||
| 'parameter', | ||
| 'uselessDefaultAssignment', | ||
| ); | ||
| } | ||
| } | ||
| } | ||
| } | ||
| } | ||
| return; | ||
| } | ||
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| if (parent.type === AST_NODE_TYPES.Property) { | ||
| const propertyType = getTypeOfProperty(parent); | ||
| if (!propertyType) { | ||
| return; | ||
| } | ||
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| if (!canBeUndefined(propertyType)) { | ||
| reportUselessDefault(node, 'property', 'uselessDefaultAssignment'); | ||
| } | ||
| } else if (parent.type === AST_NODE_TYPES.ArrayPattern) { | ||
| const sourceType = getSourceTypeForPattern(parent); | ||
| if (!sourceType) { | ||
| return; | ||
| } | ||
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| const elementIndex = parent.elements.indexOf(node); | ||
| const elementType = getArrayElementType(sourceType, elementIndex); | ||
| if (!elementType) { | ||
| return; | ||
| } | ||
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| if (!canBeUndefined(elementType)) { | ||
| reportUselessDefault(node, 'property', 'uselessDefaultAssignment'); | ||
| } | ||
| } | ||
| } | ||
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| function getTypeOfProperty(node: TSESTree.Property): ts.Type | null { | ||
| const objectPattern = node.parent as TSESTree.ObjectPattern; | ||
| const sourceType = getSourceTypeForPattern(objectPattern); | ||
| if (!sourceType) { | ||
| return null; | ||
| } | ||
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| const propertyName = getPropertyName(node.key); | ||
| if (!propertyName) { | ||
| return null; | ||
| } | ||
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| return getPropertyType(sourceType, propertyName); | ||
| } | ||
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| function getSourceTypeForPattern(pattern: TSESTree.Node): ts.Type | null { | ||
| const parent = nullThrows( | ||
| pattern.parent, | ||
| NullThrowsReasons.MissingParent, | ||
| ); | ||
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| if (parent.type === AST_NODE_TYPES.VariableDeclarator && parent.init) { | ||
| const tsNode = services.esTreeNodeToTSNodeMap.get(parent.init); | ||
| return checker.getTypeAtLocation(tsNode); | ||
| } | ||
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| if (isFunction(parent)) { | ||
| const paramIndex = parent.params.indexOf(pattern as TSESTree.Parameter); | ||
| const tsFunc = services.esTreeNodeToTSNodeMap.get(parent); | ||
| const signature = nullThrows( | ||
| checker.getSignatureFromDeclaration(tsFunc), | ||
| NullThrowsReasons.MissingToken('signature', 'function'), | ||
| ); | ||
| const params = signature.getParameters(); | ||
| return checker.getTypeOfSymbol(params[paramIndex]); | ||
| } | ||
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| if (parent.type === AST_NODE_TYPES.AssignmentPattern) { | ||
| return getSourceTypeForPattern(parent); | ||
| } | ||
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| if (parent.type === AST_NODE_TYPES.Property) { | ||
| return getTypeOfProperty(parent); | ||
| } | ||
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| if (parent.type === AST_NODE_TYPES.ArrayPattern) { | ||
| const arrayPattern = parent; | ||
| const arrayType = getSourceTypeForPattern(arrayPattern); | ||
| if (!arrayType) { | ||
| return null; | ||
| } | ||
| const elementIndex = arrayPattern.elements.indexOf( | ||
| pattern as TSESTree.DestructuringPattern, | ||
| ); | ||
| return getArrayElementType(arrayType, elementIndex); | ||
| } | ||
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| return null; | ||
| } | ||
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| function getPropertyName( | ||
| key: TSESTree.Expression | TSESTree.PrivateIdentifier, | ||
| ): string | null { | ||
| switch (key.type) { | ||
| case AST_NODE_TYPES.Identifier: | ||
| return key.name; | ||
| case AST_NODE_TYPES.Literal: | ||
| return String(key.value); | ||
| default: | ||
| return null; | ||
| } | ||
| } | ||
|
Member
There was a problem hiding this comment. [Non-Actionable] I'm surprised a utility doesn't already exist for this - but I couldn't find one.
Not requesting changes, just noting - maybe I missed one? |
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| function reportUselessDefault( | ||
| node: TSESTree.AssignmentPattern, | ||
| type: 'parameter' | 'property', | ||
| messageId: MessageId, | ||
| ): void { | ||
| context.report({ | ||
| node: node.right, | ||
| messageId, | ||
| data: { type }, | ||
| fix(fixer) { | ||
| // Remove from before the = to the end of the default value | ||
| // Find the start position (including whitespace before =) | ||
| const start = node.left.range[1]; | ||
| const end = node.range[1]; | ||
| return fixer.removeRange([start, end]); | ||
| }, | ||
| }); | ||
| } | ||
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| return { | ||
| AssignmentPattern: checkAssignmentPattern, | ||
| }; | ||
| }, | ||
| }); | ||
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