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feat(eslint-plugin): add prefer-for-of rule (#338)
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1,41 @@ | ||
| # Use for-of loops instead of standard for loops over arrays (prefer-for-of) | ||
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| This rule recommends a for-of loop when the loop index is only used to read from an array that is being iterated. | ||
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| ## Rule Details | ||
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| For cases where the index is only used to read from the array being iterated, a for-of loop is easier to read and write. | ||
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| Examples of **incorrect** code for this rule: | ||
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| ```js | ||
| for (let i = 0; i < arr.length; i++) { | ||
| console.log(arr[i]); | ||
| } | ||
| ``` | ||
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| Examples of **correct** code for this rule: | ||
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| ```js | ||
| for (const x of arr) { | ||
| console.log(x); | ||
| } | ||
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| for (let i = 0; i < arr.length; i++) { | ||
| // i is used to write to arr, so for-of could not be used. | ||
| arr[i] = 0; | ||
| } | ||
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| for (let i = 0; i < arr.length; i++) { | ||
| // i is used independent of arr, so for-of could not be used. | ||
| console.log(i, arr[i]); | ||
| } | ||
| ``` | ||
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| ## When Not To Use It | ||
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| If you transpile for browsers that do not support for-of loops, you may wish to use traditional for loops that produce more compact code. | ||
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| ## Related to | ||
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| - TSLint: ['prefer-for-of'](https://palantir.github.io/tslint/rules/prefer-for-of/) |
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1,217 @@ | ||
| import { AST_NODE_TYPES, TSESTree } from '@typescript-eslint/typescript-estree'; | ||
| import * as util from '../util'; | ||
| import { Scope } from 'ts-eslint'; | ||
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| export default util.createRule({ | ||
| name: 'prefer-for-of', | ||
| meta: { | ||
| type: 'suggestion', | ||
| docs: { | ||
| description: | ||
| 'Prefer a ‘for-of’ loop over a standard ‘for’ loop if the index is only used to access the array being iterated.', | ||
| category: 'Stylistic Issues', | ||
| recommended: false, | ||
| tslintName: 'prefer-for-of', | ||
| }, | ||
| messages: { | ||
| preferForOf: | ||
| 'Expected a `for-of` loop instead of a `for` loop with this simple iteration', | ||
| }, | ||
| schema: [], | ||
| }, | ||
| defaultOptions: [], | ||
| create(context) { | ||
| function isSingleVariableDeclaration( | ||
| node: TSESTree.Node | null, | ||
| ): node is TSESTree.VariableDeclaration { | ||
| return ( | ||
| node !== null && | ||
| node.type === AST_NODE_TYPES.VariableDeclaration && | ||
| node.kind !== 'const' && | ||
| node.declarations.length === 1 | ||
| ); | ||
| } | ||
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| function isLiteral(node: TSESTree.Expression, value: number): boolean { | ||
| return node.type === AST_NODE_TYPES.Literal && node.value === value; | ||
| } | ||
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| function isZeroInitialized(node: TSESTree.VariableDeclarator): boolean { | ||
| return node.init !== null && isLiteral(node.init, 0); | ||
| } | ||
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| function isMatchingIdentifier( | ||
| node: TSESTree.Expression, | ||
| name: string, | ||
| ): boolean { | ||
| return node.type === AST_NODE_TYPES.Identifier && node.name === name; | ||
| } | ||
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| function isLessThanLengthExpression( | ||
| node: TSESTree.Node | null, | ||
| name: string, | ||
| ): TSESTree.Expression | null { | ||
| if ( | ||
| node !== null && | ||
| node.type === AST_NODE_TYPES.BinaryExpression && | ||
| node.operator === '<' && | ||
| isMatchingIdentifier(node.left, name) && | ||
| node.right.type === AST_NODE_TYPES.MemberExpression && | ||
| isMatchingIdentifier(node.right.property, 'length') | ||
| ) { | ||
| return node.right.object; | ||
| } | ||
| return null; | ||
| } | ||
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| function isIncrement(node: TSESTree.Node | null, name: string): boolean { | ||
| if (!node) { | ||
| return false; | ||
| } | ||
| switch (node.type) { | ||
| case AST_NODE_TYPES.UpdateExpression: | ||
| // x++ or ++x | ||
| return ( | ||
| node.operator === '++' && isMatchingIdentifier(node.argument, name) | ||
| ); | ||
| case AST_NODE_TYPES.AssignmentExpression: | ||
| if (isMatchingIdentifier(node.left, name)) { | ||
| if (node.operator === '+=') { | ||
| // x += 1 | ||
| return isLiteral(node.right, 1); | ||
| } else if (node.operator === '=') { | ||
| // x = x + 1 or x = 1 + x | ||
| const expr = node.right; | ||
| return ( | ||
| expr.type === AST_NODE_TYPES.BinaryExpression && | ||
| expr.operator === '+' && | ||
| ((isMatchingIdentifier(expr.left, name) && | ||
| isLiteral(expr.right, 1)) || | ||
| (isLiteral(expr.left, 1) && | ||
| isMatchingIdentifier(expr.right, name))) | ||
| ); | ||
| } | ||
| } | ||
| } | ||
| return false; | ||
| } | ||
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| function contains(outer: TSESTree.Node, inner: TSESTree.Node): boolean { | ||
| return ( | ||
| outer.range[0] <= inner.range[0] && outer.range[1] >= inner.range[1] | ||
| ); | ||
| } | ||
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| function isAssignee(node: TSESTree.Node): boolean { | ||
| const parent = node.parent; | ||
| if (!parent) { | ||
| return false; | ||
| } | ||
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| // a[i] = 1, a[i] += 1, etc. | ||
| if ( | ||
| parent.type === AST_NODE_TYPES.AssignmentExpression && | ||
| parent.left === node | ||
| ) { | ||
| return true; | ||
| } | ||
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| // delete a[i] | ||
| if ( | ||
| parent.type === AST_NODE_TYPES.UnaryExpression && | ||
| parent.operator === 'delete' && | ||
| parent.argument === node | ||
| ) { | ||
| return true; | ||
| } | ||
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| // a[i]++, --a[i], etc. | ||
| if ( | ||
| parent.type === AST_NODE_TYPES.UpdateExpression && | ||
| parent.argument === node | ||
| ) { | ||
| return true; | ||
| } | ||
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| // [a[i]] = [0] | ||
| if (parent.type === AST_NODE_TYPES.ArrayPattern) { | ||
| return true; | ||
| } | ||
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| // [...a[i]] = [0] | ||
| if (parent.type === AST_NODE_TYPES.RestElement) { | ||
| return true; | ||
| } | ||
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| // ({ foo: a[i] }) = { foo: 0 } | ||
| if ( | ||
| parent.type === AST_NODE_TYPES.Property && | ||
| parent.parent !== undefined && | ||
| parent.parent.type === AST_NODE_TYPES.ObjectExpression && | ||
| parent.value === node && | ||
| isAssignee(parent.parent) | ||
| ) { | ||
| return true; | ||
| } | ||
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| return false; | ||
| } | ||
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| function isIndexOnlyUsedWithArray( | ||
| body: TSESTree.Statement, | ||
| indexVar: Scope.Variable, | ||
| arrayExpression: TSESTree.Expression, | ||
| ): boolean { | ||
| const sourceCode = context.getSourceCode(); | ||
| const arrayText = sourceCode.getText(arrayExpression); | ||
| return indexVar.references.every(reference => { | ||
| const id = reference.identifier; | ||
| const node = id.parent; | ||
| return ( | ||
| !contains(body, id) || | ||
| (node !== undefined && | ||
| node.type === AST_NODE_TYPES.MemberExpression && | ||
| node.property === id && | ||
| sourceCode.getText(node.object) === arrayText && | ||
| !isAssignee(node)) | ||
| ); | ||
| }); | ||
| } | ||
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| return { | ||
| 'ForStatement:exit'(node: TSESTree.ForStatement) { | ||
| if (!isSingleVariableDeclaration(node.init)) { | ||
| return; | ||
| } | ||
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| const [declarator] = node.init.declarations; | ||
| if ( | ||
| !isZeroInitialized(declarator) || | ||
| declarator.id.type !== AST_NODE_TYPES.Identifier | ||
| ) { | ||
| return; | ||
| } | ||
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| const indexName = declarator.id.name; | ||
| const arrayExpression = isLessThanLengthExpression( | ||
| node.test, | ||
| indexName, | ||
| ); | ||
| if (!arrayExpression) { | ||
| return; | ||
| } | ||
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| const [indexVar] = context.getDeclaredVariables(node.init); | ||
| if ( | ||
| isIncrement(node.update, indexName) && | ||
| isIndexOnlyUsedWithArray(node.body, indexVar, arrayExpression) | ||
| ) { | ||
| context.report({ | ||
| node, | ||
| messageId: 'preferForOf', | ||
| }); | ||
| } | ||
| }, | ||
| }; | ||
| }, | ||
| }); |
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