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Original file line number | Diff line number | Diff line change |
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# Disallow iterating over an array with a for-in loop (no-for-in-array) | ||
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This rule prohibits iterating over an array with a for-in loop. | ||
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## Rule Details | ||
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Rationale from TSLint: | ||
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A for-in loop (`for (var k in o)`) iterates over the properties of an Object. | ||
While it is legal to use for-in loops with array types, it is not common. | ||
for-in will iterate over the indices of the array as strings, omitting any "holes" in | ||
the array. | ||
More common is to use for-of, which iterates over the values of an array. | ||
If you want to iterate over the indices, alternatives include: | ||
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```js | ||
array.forEach((value, index) => { ... }); | ||
for (const [index, value] of array.entries()) { ... } | ||
for (let i = 0; i < array.length; i++) { ... } | ||
``` | ||
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Examples of **incorrect** code for this rule: | ||
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```js | ||
for (const x in [3, 4, 5]) { | ||
console.log(x); | ||
} | ||
``` | ||
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Examples of **correct** code for this rule: | ||
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```js | ||
for (const x in { a: 3, b: 4, c: 5 }) { | ||
console.log(x); | ||
} | ||
``` | ||
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## When Not To Use It | ||
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If you want to iterate through a loop using the indices in an array as strings, you can turn off this rule. | ||
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## Related to | ||
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- TSLint: ['no-for-in-array'](https://palantir.github.io/tslint/rules/no-for-in-array/) |
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/** | ||
* @fileoverview Disallow iterating over an array with a for-in loop | ||
* @author Benjamin Lichtman | ||
*/ | ||
'use strict'; | ||
const ts = require('typescript'); | ||
const util = require('../util'); | ||
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//------------------------------------------------------------------------------ | ||
// Rule Definition | ||
//------------------------------------------------------------------------------ | ||
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/** | ||
* @type {import("eslint").Rule.RuleModule} | ||
*/ | ||
module.exports = { | ||
meta: { | ||
docs: { | ||
description: 'Disallow iterating over an array with a for-in loop', | ||
category: 'Functionality', | ||
recommended: false, | ||
extraDescription: [util.tslintRule('no-for-in-array')], | ||
url: util.metaDocsUrl('no-for-in-array') | ||
}, | ||
fixable: null, | ||
messages: { | ||
forInViolation: | ||
'For-in loops over arrays are forbidden. Use for-of or array.forEach instead.' | ||
}, | ||
schema: [], | ||
type: 'problem' | ||
}, | ||
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create(context) { | ||
return { | ||
ForInStatement(node) { | ||
const parserServices = util.getParserServices(context); | ||
const checker = parserServices.program.getTypeChecker(); | ||
const originalNode = parserServices.esTreeNodeToTSNodeMap.get(node); | ||
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const type = checker.getTypeAtLocation(originalNode.expression); | ||
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if ( | ||
(typeof type.symbol !== 'undefined' && | ||
type.symbol.name === 'Array') || | ||
(type.flags & ts.TypeFlags.StringLike) !== 0 | ||
) { | ||
context.report({ | ||
node, | ||
messageId: 'forInViolation' | ||
}); | ||
} | ||
} | ||
}; | ||
} | ||
}; |
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Original file line number | Diff line number | Diff line change |
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/** | ||
* @fileoverview Disallow iterating over an array with a for-in loop | ||
* @author Benjamin Lichtman | ||
*/ | ||
'use strict'; | ||
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//------------------------------------------------------------------------------ | ||
// Requirements | ||
//------------------------------------------------------------------------------ | ||
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const rule = require('../../../lib/rules/no-for-in-array'), | ||
RuleTester = require('eslint').RuleTester, | ||
path = require('path'); | ||
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//------------------------------------------------------------------------------ | ||
// Tests | ||
//------------------------------------------------------------------------------ | ||
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const rootDir = path.join(process.cwd(), 'tests/fixtures/'); | ||
const parserOptions = { | ||
ecmaVersion: 2015, | ||
tsconfigRootDir: rootDir, | ||
project: './tsconfig.json' | ||
}; | ||
const ruleTester = new RuleTester({ | ||
parserOptions, | ||
parser: '@typescript-eslint/parser' | ||
}); | ||
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ruleTester.run('no-for-in-array', rule, { | ||
valid: [ | ||
` | ||
for (const x of [3, 4, 5]) { | ||
console.log(x); | ||
}`, | ||
` | ||
for (const x in { a: 1, b: 2, c: 3 }) { | ||
console.log(x); | ||
}` | ||
], | ||
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invalid: [ | ||
{ | ||
code: ` | ||
for (const x in [3, 4, 5]) { | ||
console.log(x); | ||
}`, | ||
errors: [ | ||
{ | ||
messageId: 'forInViolation', | ||
type: 'ForInStatement' | ||
} | ||
] | ||
}, | ||
{ | ||
code: ` | ||
const z = [3, 4, 5]; | ||
for (const x in z) { | ||
console.log(x); | ||
}`, | ||
errors: [ | ||
{ | ||
messageId: 'forInViolation', | ||
type: 'ForInStatement' | ||
} | ||
] | ||
} | ||
] | ||
}); |
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