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Add no-new-array rule (#992)
Co-authored-by: Sindre Sorhus <[email protected]>
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‎docs/rules/no-new-array.md‎

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# Disallow `new Array()`
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The ESLint built-in rule [`no-array-constructor`](https://eslint.org/docs/rules/no-array-constructor) enforces using an array literal instead of the `Array` constructor, but it still allows using the `Array` constructor with **one** argument. This rule fills that gap.
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When using the `Array` constructor with one argument, it's not clear whether the argument is meant to be the length of the array or the only element.
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This rule is fixable if the value type of the argument is known.
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## Fail
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```js
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const array = new Array(length);
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```
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```js
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const array = new Array(onlyElement);
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```
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```js
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const array = new Array(...unknownArgumentsList);
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```
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## Pass
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```js
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const array = Array.from({length});
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```
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```js
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const array = [onlyElement];
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```
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```js
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const array = [...items];
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```

‎index.js‎

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@@ -64,6 +64,7 @@ module.exports = {
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'unicorn/no-lonely-if': 'error',
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'no-nested-ternary': 'off',
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'unicorn/no-nested-ternary': 'error',
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'unicorn/no-new-array': 'error',
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'unicorn/no-new-buffer': 'error',
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'unicorn/no-null': 'error',
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'unicorn/no-object-as-default-parameter': 'error',

‎readme.md‎

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@@ -58,6 +58,7 @@ Configure it in `package.json`.
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"unicorn/no-lonely-if": "error",
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"no-nested-ternary": "off",
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"unicorn/no-nested-ternary": "error",
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"unicorn/no-new-array": "error",
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"unicorn/no-new-buffer": "error",
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"unicorn/no-null": "error",
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"unicorn/no-object-as-default-parameter": "error",
@@ -130,6 +131,7 @@ Configure it in `package.json`.
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- [no-keyword-prefix](docs/rules/no-keyword-prefix.md) - Disallow identifiers starting with `new` or `class`.
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- [no-lonely-if](docs/rules/no-lonely-if.md) - Disallow `if` statements as the only statement in `if` blocks without `else`. *(fixable)*
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- [no-nested-ternary](docs/rules/no-nested-ternary.md) - Disallow nested ternary expressions. *(partly fixable)*
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- [no-new-array](docs/rules/no-new-array.md) - Disallow `new Array()`. *(partly fixable)*
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- [no-new-buffer](docs/rules/no-new-buffer.md) - Enforce the use of `Buffer.from()` and `Buffer.alloc()` instead of the deprecated `new Buffer()`. *(fixable)*
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- [no-null](docs/rules/no-null.md) - Disallow the use of the `null` literal.
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- [no-object-as-default-parameter](docs/rules/no-object-as-default-parameter.md) - Disallow the use of objects as default parameters.

‎rules/no-new-array.js‎

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'use strict';
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const {isParenthesized, getStaticValue} = require('eslint-utils');
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const getDocumentationUrl = require('./utils/get-documentation-url');
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const needsSemicolon = require('./utils/needs-semicolon');
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const MESSAGE_ID_ERROR = 'error';
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const MESSAGE_ID_LENGTH = 'array-length';
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const MESSAGE_ID_ONLY_ELEMENT = 'only-element';
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const MESSAGE_ID_SPREAD = 'spread';
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const messages = {
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[MESSAGE_ID_ERROR]: 'Do not use `new Array()`.',
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[MESSAGE_ID_LENGTH]: 'The argument is the length of array.',
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[MESSAGE_ID_ONLY_ELEMENT]: 'The argument is the only element of array.',
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[MESSAGE_ID_SPREAD]: 'Spread the argument.'
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};
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const newArraySelector = [
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'NewExpression',
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'[callee.type="Identifier"]',
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'[callee.name="Array"]',
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'[arguments.length=1]'
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].join('');
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function getProblem(context, node) {
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const problem = {
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node,
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messageId: MESSAGE_ID_ERROR
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};
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const [argumentNode] = node.arguments;
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const sourceCode = context.getSourceCode();
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let text = sourceCode.getText(argumentNode);
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if (isParenthesized(argumentNode, sourceCode)) {
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text = `(${text})`;
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}
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const maybeSemiColon = needsSemicolon(sourceCode.getTokenBefore(node), sourceCode, '[') ?
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';' :
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'';
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// We are not sure how many `arguments` passed
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if (argumentNode.type === 'SpreadElement') {
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problem.suggest = [
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{
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messageId: MESSAGE_ID_SPREAD,
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fix: fixer => fixer.replaceText(node, `${maybeSemiColon}[${text}]`)
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}
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];
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return problem;
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}
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const result = getStaticValue(argumentNode, context.getScope());
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const fromLengthText = `Array.from(${text === 'length' ? '{length}' : `{length: ${text}}`})`;
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const onlyElementText = `${maybeSemiColon}[${text}]`;
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// We don't know the argument is number or not
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if (result === null) {
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problem.suggest = [
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{
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messageId: MESSAGE_ID_LENGTH,
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fix: fixer => fixer.replaceText(node, fromLengthText)
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},
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{
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messageId: MESSAGE_ID_ONLY_ELEMENT,
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fix: fixer => fixer.replaceText(node, onlyElementText)
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}
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];
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return problem;
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}
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problem.fix = fixer => fixer.replaceText(
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node,
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typeof result.value === 'number' ? fromLengthText : onlyElementText
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);
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return problem;
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}
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const create = context => ({
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[newArraySelector](node) {
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context.report(getProblem(context, node));
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}
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});
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module.exports = {
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create,
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meta: {
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type: 'suggestion',
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docs: {
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url: getDocumentationUrl(__filename)
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},
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fixable: 'code',
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messages
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}
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};

‎test/no-new-array.js‎

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import {outdent} from 'outdent';
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import {test} from './utils/test';
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const MESSAGE_ID_ERROR = 'error';
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const MESSAGE_ID_LENGTH = 'array-length';
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const MESSAGE_ID_ONLY_ELEMENT = 'only-element';
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const MESSAGE_ID_SPREAD = 'spread';
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const suggestionCase = ({code, suggestions}) => ({
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code,
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output: code,
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errors: [
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{
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messageId: MESSAGE_ID_ERROR,
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suggestions
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}
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]
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});
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test({
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valid: [
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'const array = Array.from({length: 1})',
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// ESLint builtin rule `no-array-constructor` cases
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'const array = new Array()',
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'const array = new Array',
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'const array = new Array(1, 2)',
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'const array = Array(1, 2)',
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// `unicorn/new-for-builtins` cases
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'const array = Array(1)'
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],
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invalid: [
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suggestionCase({
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code: 'const array = new Array(foo)',
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suggestions: [
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{
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messageId: MESSAGE_ID_LENGTH,
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output: 'const array = Array.from({length: foo})'
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},
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{
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messageId: MESSAGE_ID_ONLY_ELEMENT,
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output: 'const array = [foo]'
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}
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]
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}),
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suggestionCase({
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code: 'const array = new Array(length)',
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suggestions: [
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{
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messageId: MESSAGE_ID_LENGTH,
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output: 'const array = Array.from({length})'
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},
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{
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messageId: MESSAGE_ID_ONLY_ELEMENT,
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output: 'const array = [length]'
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}
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]
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}),
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suggestionCase({
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code: outdent`
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const foo = []
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new Array(bar).forEach(baz)
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`,
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suggestions: [
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{
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messageId: MESSAGE_ID_LENGTH,
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output: outdent`
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const foo = []
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Array.from({length: bar}).forEach(baz)
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`
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},
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{
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messageId: MESSAGE_ID_ONLY_ELEMENT,
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output: outdent`
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const foo = []
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;[bar].forEach(baz)
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`
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}
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]
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}),
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...[
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'...[foo]',
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'...foo',
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'...[...foo]',
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// The following cases we can know the result, but we are not auto-fixing them
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'...[1]',
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'...["1"]',
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'...[1, "1"]'
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].map(argumentText => {
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const code = `const array = new Array(${argumentText})`;
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return {
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code,
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output: code,
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errors: [
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{
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messageId: MESSAGE_ID_ERROR,
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suggestions: [
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{
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messageId: MESSAGE_ID_SPREAD,
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output: `const array = [${argumentText}]`
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}
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]
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}
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]
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};
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}),
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suggestionCase({
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code: outdent`
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const foo = []
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new Array(...bar).forEach(baz)
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`,
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suggestions: [
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{
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messageId: MESSAGE_ID_SPREAD,
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output: outdent`
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const foo = []
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;[...bar].forEach(baz)
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`
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}
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]
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})
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]
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});
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test.visualize([
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'const array = new Array(1)',
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// This is actually `[]`, but we fix to `Array.from({length: zero})`
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outdent`
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const zero = 0;
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const array = new Array(zero);
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`,
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// Use shorthand
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outdent`
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const length = 1;
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const array = new Array(length);
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`,
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'const array = new Array(1.5)',
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'const array = new Array(Number("1"))',
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'const array = new Array("1")',
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'const array = new Array(null)',
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'const array = new Array(("1"))',
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'const array = new Array((0, 1))',
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outdent`
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const foo = []
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new Array("bar").forEach(baz)
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`
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]);

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