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Add prefer-iterator-to-array-at-end rule (#3041)
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‎docs/rules/prefer-iterator-to-array-at-end.md‎

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# prefer-iterator-to-array-at-end
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📝 Prefer moving `.toArray()` to the end of iterator helper chains.
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💼 This rule is enabled in the following [configs](https://github.com/sindresorhus/eslint-plugin-unicorn#recommended-config): ✅ `recommended`, ☑️ `unopinionated`.
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💡 This rule is manually fixable by [editor suggestions](https://eslint.org/docs/latest/use/core-concepts#rule-suggestions).
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<!-- end auto-generated rule header -->
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Prefer moving `Iterator#toArray()` to the end of iterator helper chains.
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Iterator helpers are lazy. Calling `toArray()` before helper methods like `map()` or `filter()` creates a temporary array and makes the rest of the chain use `Array` methods instead of lazy `Iterator` methods.
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## Examples
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```js
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// ❌
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const result = iterator.toArray().map(fn);
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// ✅
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const result = iterator.map(fn).toArray();
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```
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```js
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// ❌
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const result = iterator.toArray().filter(fn);
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// ✅
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const result = iterator.filter(fn).toArray();
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```
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Cases are reported as suggestions instead of autofixes because moving `toArray()` changes when callbacks run: `Array` methods run after the iterator has been exhausted, while `Iterator` helpers run lazily as the result is consumed. `Array` callbacks also receive an extra `array` argument that `Iterator` callbacks do not.
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`flatMap()` has an additional difference: `Array#flatMap()` accepts non-iterable callback results, while `Iterator#flatMap()` requires iterable results.
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```js
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// ❌
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const result = iterator.toArray().flatMap(fn);
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// ✅
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const result = iterator.flatMap(fn).toArray();
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```
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This rule only handles direct lazy helper equivalents. It intentionally does not convert `Array#slice()` to `Iterator#take()` or `Iterator#drop()`.

‎readme.md‎

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| [prefer-import-meta-properties](docs/rules/prefer-import-meta-properties.md) | Prefer `import.meta.{dirname,filename}` over legacy techniques for getting file paths. | | 🔧 | |
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| [prefer-includes](docs/rules/prefer-includes.md) | Prefer `.includes()` over `.indexOf()`, `.lastIndexOf()`, and `Array#some()` when checking for existence or non-existence. | ✅ ☑️ | 🔧 | 💡 |
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| [prefer-iterator-concat](docs/rules/prefer-iterator-concat.md) | Prefer `Iterator.concat(…)` over temporary spread arrays. | | 🔧 | 💡 |
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| [prefer-iterator-to-array-at-end](docs/rules/prefer-iterator-to-array-at-end.md) | Prefer moving `.toArray()` to the end of iterator helper chains. | ✅ ☑️ | | 💡 |
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| [prefer-keyboard-event-key](docs/rules/prefer-keyboard-event-key.md) | Prefer `KeyboardEvent#key` over deprecated keyboard event properties. | ✅ ☑️ | 🔧 | 💡 |
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| [prefer-logical-operator-over-ternary](docs/rules/prefer-logical-operator-over-ternary.md) | Prefer using a logical operator over a ternary. | ✅ ☑️ | | 💡 |
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| [prefer-math-abs](docs/rules/prefer-math-abs.md) | Prefer `Math.abs()` over manual absolute value expressions and symmetric range checks. | ✅ ☑️ | 🔧 | |

‎rules/index.js‎

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export {default as 'prefer-import-meta-properties'} from './prefer-import-meta-properties.js';
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export {default as 'prefer-includes'} from './prefer-includes.js';
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export {default as 'prefer-iterator-concat'} from './prefer-iterator-concat.js';
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export {default as 'prefer-iterator-to-array-at-end'} from './prefer-iterator-to-array-at-end.js';
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export {default as 'prefer-keyboard-event-key'} from './prefer-keyboard-event-key.js';
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export {default as 'prefer-logical-operator-over-ternary'} from './prefer-logical-operator-over-ternary.js';
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export {default as 'prefer-math-abs'} from './prefer-math-abs.js';

‎rules/prefer-iterator-to-array-at-end.js‎

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import {
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getCallExpressionArgumentsText,
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getCallExpressionTokens,
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getParenthesizedText,
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} from './utils/index.js';
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import {isMethodCall} from './ast/index.js';
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const MESSAGE_ID = 'prefer-iterator-to-array-at-end';
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const methods = [
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'filter',
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'flatMap',
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'map',
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];
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const messages = {
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[MESSAGE_ID]: 'Move `.toArray()` after `.{{method}}(…)`.',
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};
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const isToArrayCall = node => isMethodCall(node, {
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method: 'toArray',
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argumentsLength: 0,
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optionalCall: false,
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optionalMember: false,
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});
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const hasArrayParameter = node => (
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node.type === 'ArrowFunctionExpression'
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|| node.type === 'FunctionExpression'
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) && (
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node.params.length > 2
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|| node.params.some(parameter => parameter.type === 'RestElement')
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);
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const getReplacementText = (toArrayCall, methodCall, context, openingParenthesisToken) => {
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const {sourceCode} = context;
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const iteratorText = getParenthesizedText(toArrayCall.callee.object, context);
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const method = methodCall.callee.property.name;
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const argumentsText = getCallExpressionArgumentsText(context, methodCall);
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const [, methodNameEnd] = sourceCode.getRange(methodCall.callee.property);
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const [openingParenthesisStart] = sourceCode.getRange(openingParenthesisToken);
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const textBetweenMethodAndArguments = sourceCode.text.slice(methodNameEnd, openingParenthesisStart);
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return `${iteratorText}.${method}${textBetweenMethodAndArguments}(${argumentsText}).toArray()`;
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};
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const getFix = (toArrayCall, methodCall, context) => {
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const {sourceCode} = context;
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const {
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openingParenthesisToken,
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closingParenthesisToken,
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} = getCallExpressionTokens(methodCall, context);
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const [, methodNameEnd] = sourceCode.getRange(methodCall.callee.property);
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const [argumentsEnd] = sourceCode.getRange(closingParenthesisToken);
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if (sourceCode.getCommentsInside(methodCall).some(comment => {
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const [start, end] = sourceCode.getRange(comment);
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return start < methodNameEnd || end > argumentsEnd;
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})) {
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return;
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}
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return fixer => fixer.replaceText(methodCall, getReplacementText(toArrayCall, methodCall, context, openingParenthesisToken));
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};
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/** @param {import('eslint').Rule.RuleContext} context */
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const create = context => {
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context.on('CallExpression', node => {
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if (
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!isMethodCall(node, {
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methods,
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argumentsLength: 1,
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optionalCall: false,
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optionalMember: false,
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})
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|| !isToArrayCall(node.callee.object)
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|| hasArrayParameter(node.arguments[0])
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) {
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return;
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}
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const method = node.callee.property.name;
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const toArrayCall = node.callee.object;
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const fix = getFix(toArrayCall, node, context);
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const problem = {
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node: toArrayCall.callee.property,
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messageId: MESSAGE_ID,
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data: {method},
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};
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if (!fix) {
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return problem;
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}
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return {
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...problem,
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suggest: [
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{
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messageId: MESSAGE_ID,
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data: {method},
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fix,
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},
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],
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};
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});
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};
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/** @type {import('eslint').Rule.RuleModule} */
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const config = {
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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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description: 'Prefer moving `.toArray()` to the end of iterator helper chains.',
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recommended: 'unopinionated',
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},
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hasSuggestions: true,
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messages,
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},
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};
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export default config;

‎test/prefer-iterator-to-array-at-end.js‎

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import outdent from 'outdent';
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import {getTester, parsers} from './utils/test.js';
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const {test} = getTester(import.meta);
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test.snapshot({
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valid: [
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// Already at the end.
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'iterator.map(fn).toArray()',
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'iterator.filter(fn).toArray()',
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'iterator.flatMap(fn).toArray()',
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// Not `.toArray()`.
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'iterator.map(fn)',
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'iterator.filter(fn)',
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'array.toSorted().map(fn)',
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// `.toArray()` with arguments.
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'iterator.toArray(true).map(fn)',
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'iterator.toArray(true).filter(fn)',
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'iterator.toArray(true).flatMap(fn)',
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// Optional chaining.
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'iterator?.toArray().map(fn)',
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'iterator.toArray?.().map(fn)',
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'iterator.toArray()?.map(fn)',
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'iterator.toArray().map?.(fn)',
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// Computed properties.
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'iterator["toArray"]().map(fn)',
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'iterator.toArray()["map"](fn)',
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// Extra arguments.
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'iterator.toArray().map(fn, thisArgument)',
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'iterator.toArray().filter(fn, thisArgument)',
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'iterator.toArray().flatMap(fn, thisArgument)',
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'iterator.toArray().map(...arguments_)',
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// Array callbacks receive the 3rd `array` argument, Iterator callbacks do not.
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'iterator.toArray().map((value, index, array) => array.length)',
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'iterator.toArray().filter(function (value, index, array) { return array.length; })',
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'iterator.toArray().flatMap((value, index, array) => array)',
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'iterator.toArray().map((...values) => values.length)',
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// Other methods are handled by other rules or intentionally ignored.
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'iterator.toArray().every(fn)',
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'iterator.toArray().find(fn)',
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'iterator.toArray().forEach(fn)',
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'iterator.toArray().reduce(fn, init)',
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'iterator.toArray().some(fn)',
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'iterator.toArray().slice(1)',
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'iterator.toArray().sort()',
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'iterator.toArray().at(0)',
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'iterator.toArray().length',
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],
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invalid: [
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'iterator.toArray().map(fn)',
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'iterator.toArray().filter(fn)',
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'iterator.toArray().flatMap(fn)',
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'iterator.toArray().filter(function (value) { return value; })',
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// Inline callbacks.
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'iterator.toArray().map(value => value * 2)',
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'iterator.toArray().filter((value, index) => index > 0)',
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'iterator.toArray().flatMap(value => [value])',
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'iterator.toArray().flatMap(value => value)',
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// Parenthesized.
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'(iterator.toArray()).map(fn)',
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'((iterator.toArray())).filter(fn)',
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'((iterator).toArray()).flatMap(fn)',
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// Complex receivers.
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'getIterator().toArray().map(fn)',
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'(condition ? iterator : otherIterator).toArray().filter(fn)',
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// Chained methods are handled one helper at a time.
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'iterator.toArray().map(mapper).filter(predicate)',
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// Comments inside the helper call arguments are preserved.
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'iterator.toArray().map(/* comment */ fn)',
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// Comments inside `.toArray()` are reported without a fix.
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'iterator.toArray(/* comment */).map(fn)',
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// Comments between `.toArray()` and the helper method are reported without a fix.
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'iterator.toArray() /* comment */ .map(value => value)',
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// Comments before helper arguments are preserved.
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'iterator.toArray().map /* comment */ (value => value)',
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// Multiline.
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outdent`
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const result = iterator
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.toArray()
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.map(value => value * 2);
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`,
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// TypeScript.
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{
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code: '(iterator as Iterator<number>).toArray().map(value => value * 2)',
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languageOptions: {
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parser: parsers.typescript,
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},
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},
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{
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code: 'iterator.toArray().map<string>(value => value)',
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languageOptions: {
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parser: parsers.typescript,
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},
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},
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{
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code: 'iterator!.toArray().filter(Boolean)',
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languageOptions: {
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parser: parsers.typescript,
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},
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},
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],
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});

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