8000 feat(eslint-plugin) [prefer-at] Create rule #6401 · MillerSvt/typescript-eslint@125a83f · GitHub
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sviat9440Святослав Зайцев
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feat(eslint-plugin) [prefer-at] Create rule typescript-eslint#6401 
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‎packages/eslint-plugin/docs/rules/prefer-at.md

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---
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description: Enforce the use of `array.at(-1)` instead of `array[array.length - 1]`
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---
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> 🛑 This file is source code, not the primary documentation location! 🛑
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>
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> See **https://typescript-eslint.io/rules/prefer-at** for documentation.
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There are two ways to get the last item of the array:
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- `arr[arr.length - 1]`: getting an item by index calculated relative to the length of the array.
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- `arr.at(-1)`: getting an item by the `at` method. [Docs](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/at)
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`arr.at(-1)` is a cleaner equivalent to `arr[arr.length - 1]`.
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## Examples
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<!--tabs-->
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### ❌ Incorrect
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```ts
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let arr = [1, 2, 3];
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let a = arr[arr.length - 1];
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```
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### ✅ Correct
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```ts
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let arr = [1, 2, 3];
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let a = arr.at(-1);
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```
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<!--/tabs-->
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## When Not To Use It
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If you support a browser that does not match
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[this table](https://caniuse.com/mdn-javascript_builtins_array_at)
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or use `node.js < 16.6.0` and don't include the polyfill

‎packages/eslint-plugin/src/rules/index.ts

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@@ -94,6 +94,7 @@ import objectCurlySpacing from './object-curly-spacing';
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import paddingLineBetweenStatements from './padding-line-between-statements';
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import parameterProperties from './parameter-properties';
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import preferAsConst from './prefer-as-const';
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import preferAt from './prefer-at';
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import preferEnumInitializers from './prefer-enum-initializers';
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import preferForOf from './prefer-for-of';
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import preferFunctionType from './prefer-function-type';
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'padding-line-between-statements': paddingLineBetweenStatements,
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'parameter-properties': parameterProperties,
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'prefer-as-const': preferAsConst,
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'prefer-at': preferAt,
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'prefer-enum-initializers': preferEnumInitializers,
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'prefer-for-of': preferForOf,
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'prefer-function-type': preferFunctionType,

‎packages/eslint-plugin/src/rules/prefer-at.ts

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import type { TSESTree } from '@typescript-eslint/utils';
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import { AST_NODE_TYPES } from '@typescript-eslint/utils';
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import * as util from '../util';
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export default util.createRule({
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name: 'prefer-at',
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meta: {
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type: 'suggestion',
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fixable: 'code',
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docs: {
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description:
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'Enforce the use of `array.at(-1)` instead of `array[array.length - 1]`',
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recommended: false,
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},
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messages: {
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preferAt:
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'Expected a `{{name}}.at(-1)` instead of `{{name}}[{{name}}.length - 1]`.',
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},
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schema: [],
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},
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defaultOptions: [],
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create(context) {
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class UnknownNodeError extends Error {
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public constructor(node: TSESTree.Node) {
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super(`UnknownNode ${node.type}`);
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}
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}
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function getName(node: TSESTree.Node): string {
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switch (node.type) {
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case AST_NODE_TYPES.PrivateIdentifier:
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return `#${node.name}`;
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case AST_NODE_TYPES.Identifier:
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return node.name;
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case AST_NODE_TYPES.ThisExpression:
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return 'this';
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case AST_NODE_TYPES.MemberExpression:
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return `${getName(node.object)}.${getName(node.property)}`;
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default:
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throw new UnknownNodeError(node);
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}
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}
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return {
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MemberExpression(node: TSESTree.MemberExpression): void {
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try {
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if (
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node.property.type !== AST_NODE_TYPES.BinaryExpression ||
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node.property.operator !== '-' ||
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node.property.right.type !== AST_NODE_TYPES.Literal ||
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node.property.right.value !== 1
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) {
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return;
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}
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const objectName = getName(node.object);
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const propertyLeftName = getName(node.property.left);
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if (`${objectName}.length` === propertyLeftName) {
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context.report({
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messageId: 'preferAt',
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data: {
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name: objectName,
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},
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node,
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fix: fixer => fixer.replaceText(node, `${objectName}.at(-1)`),
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});
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}
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} catch (error) {
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if (error instanceof UnknownNodeError) {
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return;
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}
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throw error;
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}
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},
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};
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},
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});

‎packages/eslint-plugin/tests/rules/prefer-at.test.ts

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import rule from '../../src/rules/prefer-at';
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import { RuleTester } from '../RuleTester';
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const ruleTester = new RuleTester({
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parser: '@typescript-eslint/parser',
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});
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ruleTester.run('prefer-at', rule, {
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valid: [
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'const a = arr.at(-1);',
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'const a = this.arr.at(-1);',
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'const a = this.#arr.at(-1);',
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'const a = this.#prop.arr.at(-1);',
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'const a = arr[arr.length + 1];',
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'const a = (arr ? b : c)[arr.length - 1];',
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],
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invalid: [
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{
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code: 'const a = arr[arr.length - 1];',
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errors: [
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{
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messageId: 'preferAt',
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data: {
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name: 'arr',
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},
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},
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],
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output: 'const a = arr.at(-1);',
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},
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{
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code: 'const a = this.arr[this.arr.length - 1];',
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errors: [
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{
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messageId: 'preferAt',
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data: {
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name: 'this.arr',
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},
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},
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],
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output: 'const a = this.arr.at(-1);',
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},
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{
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code: 'const a = this.#arr[this.#arr.length - 1];',
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errors: [
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{
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messageId: 'preferAt',
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data: {
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name: 'this.#arr',
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},
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},
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],
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output: 'const a = this.#arr.at(-1);',
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},
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{
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code: 'const a = this.#prop.arr[this.#prop.arr.length - 1];',
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errors: [
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{
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messageId: 'preferAt',
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data: {
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name: 'this.#prop.arr',
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},
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},
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],
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output: 'const a = this.#prop.arr.at(-1);',
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},
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],
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});

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