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fiskerfisker
authoredMay 24, 2024··
Add no-negation-in-equality-check rule (#2353)
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v58.0.0v54.0.0
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‎docs/rules/no-negation-in-equality-check.md

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# Disallow negated expression in equality check
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💼 This rule is enabled in the ✅ `recommended` [config](https://github.com/sindresorhus/eslint-plugin-unicorn#preset-configs-eslintconfigjs).
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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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<!-- Do not manually modify this header. Run: `npm run fix:eslint-docs` -->
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Using a negated expression in equality check is most likely a mistake.
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## Fail
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```js
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if (!foo === bar) {}
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```
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```js
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if (!foo !== bar) {}
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```
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## Pass
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```js
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if (foo !== bar) {}
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```
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```js
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if (!(foo === bar)) {}
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```

‎readme.md

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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`. | ✅ | 🔧 | |
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| [no-magic-array-flat-depth](docs/rules/no-magic-array-flat-depth.md) | Disallow a magic number as the `depth` argument in `Array#flat(…).` | ✅ | | |
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| [no-negated-condition](docs/rules/no-negated-condition.md) | Disallow negated conditions. | ✅ | 🔧 | |
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| [no-negation-in-equality-check](docs/rules/no-negation-in-equality-check.md) | Disallow negated expression in equality check. | ✅ | | 💡 |
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| [no-nested-ternary](docs/rules/no-nested-ternary.md) | Disallow nested ternary expressions. | ✅ | 🔧 | |
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| [no-new-array](docs/rules/no-new-array.md) | Disallow `new Array()`. | ✅ | 🔧 | 💡 |
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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()`. | ✅ | 🔧 | 💡 |

‎rules/no-negation-in-equality-check.js

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'use strict';
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const {
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fixSpaceAroundKeyword,
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addParenthesizesToReturnOrThrowExpression,
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} = require('./fix/index.js');
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const {
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needsSemicolon,
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isParenthesized,
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isOnSameLine,
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} = require('./utils/index.js');
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const MESSAGE_ID_ERROR = 'no-negation-in-equality-check/error';
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const MESSAGE_ID_SUGGESTION = 'no-negation-in-equality-check/suggestion';
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const messages = {
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[MESSAGE_ID_ERROR]: 'Negated expression in not allowed in equality check.',
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[MESSAGE_ID_SUGGESTION]: 'Switch to \'{{operator}}\' check.',
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};
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const EQUALITY_OPERATORS = new Set([
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'===',
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'!==',
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'==',
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'!=',
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]);
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const isEqualityCheck = node => node.type === 'BinaryExpression' && EQUALITY_OPERATORS.has(node.operator);
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const isNegatedExpression = node => node.type === 'UnaryExpression' && node.prefix && node.operator === '!';
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/** @param {import('eslint').Rule.RuleContext} context */
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const create = context => ({
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BinaryExpression(binaryExpression) {
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const {operator, left} = binaryExpression;
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if (
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!isEqualityCheck(binaryExpression)
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|| !isNegatedExpression(left)
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) {
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return;
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}
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const {sourceCode} = context;
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const bangToken = sourceCode.getFirstToken(left);
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const negatedOperator = `${operator.startsWith('!') ? '=' : '!'}${operator.slice(1)}`;
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return {
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node: bangToken,
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messageId: MESSAGE_ID_ERROR,
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/** @param {import('eslint').Rule.RuleFixer} fixer */
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suggest: [
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{
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messageId: MESSAGE_ID_SUGGESTION,
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data: {
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operator: negatedOperator,
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},
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/** @param {import('eslint').Rule.RuleFixer} fixer */
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* fix(fixer) {
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yield * fixSpaceAroundKeyword(fixer, binaryExpression, sourceCode);
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const tokenAfterBang = sourceCode.getTokenAfter(bangToken);
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const {parent} = binaryExpression;
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if (
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(parent.type === 'ReturnStatement' || parent.type === 'ThrowStatement')
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&& !isParenthesized(binaryExpression, sourceCode)
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) {
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const returnToken = sourceCode.getFirstToken(parent);
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if (!isOnSameLine(returnToken, tokenAfterBang)) {
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yield * addParenthesizesToReturnOrThrowExpression(fixer, parent, sourceCode);
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}
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}
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yield fixer.remove(bangToken);
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const previousToken = sourceCode.getTokenBefore(bangToken);
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if (needsSemicolon(previousToken, sourceCode, tokenAfterBang.value)) {
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yield fixer.insertTextAfter(bangToken, ';');
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}
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const operatorToken = sourceCode.getTokenAfter(
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left,
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token => token.type === 'Punctuator' && token.value === operator,
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);
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yield fixer.replaceText(operatorToken, negatedOperator);
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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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/** @type {import('eslint').Rule.RuleModule} */
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module.exports = {
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create,
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meta: {
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type: 'problem',
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docs: {
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description: 'Disallow negated expression in equality check.',
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recommended: true,
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},
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hasSuggestions: true,
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messages,
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},
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};

‎test/no-negation-in-equality-check.mjs

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import outdent from 'outdent';
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import {getTester} from './utils/test.mjs';
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const {test} = getTester(import.meta);
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test.snapshot({
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valid: [
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'!foo instanceof bar',
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'+foo === bar',
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'!(foo === bar)',
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// We are not checking right side
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'foo === !bar',
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],
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invalid: [
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'!foo === bar',
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'!foo !== bar',
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'!foo == bar',
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'!foo != bar',
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outdent`
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function x() {
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return!foo === bar;
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}
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`,
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outdent`
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function x() {
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return!
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foo === bar;
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throw!
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foo === bar;
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}
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`,
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outdent`
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foo
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!(a) === b
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`,
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outdent`
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foo
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![a, b].join('') === c
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`,
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outdent`
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foo
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! [a, b].join('') === c
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`,
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outdent`
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foo
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!/* comment */[a, b].join('') === c
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`,
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'!!foo === bar',
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],
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});

‎test/snapshots/no-negation-in-equality-check.mjs.md

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# Snapshot report for `test/no-negation-in-equality-check.mjs`
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The actual snapshot is saved in `no-negation-in-equality-check.mjs.snap`.
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Generated by [AVA](https://avajs.dev).
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## invalid(1): !foo === bar
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> Input
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`␊
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1 | !foo === bar␊
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`
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> Error 1/1
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`␊
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> 1 | !foo === bar␊
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| ^ Negated expression in not allowed in equality check.␊
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␊
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--------------------------------------------------------------------------------␊
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Suggestion 1/1: Switch to '!==' check.␊
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1 | foo !== bar␊
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`
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## invalid(2): !foo !== bar
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> Input
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`␊
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1 | !foo !== bar␊
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`
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> Error 1/1
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`␊
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> 1 | !foo !== bar␊
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| ^ Negated expression in not allowed in equality check.␊
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␊
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--------------------------------------------------------------------------------␊
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Suggestion 1/1: Switch to '===' check.␊
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1 | foo === bar␊
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`
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## invalid(3): !foo == bar
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> Input
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`␊
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1 | !foo == bar␊
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`
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> Error 1/1
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`␊
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> 1 | !foo == bar␊
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| ^ Negated expression in not allowed in equality check.␊
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␊
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--------------------------------------------------------------------------------␊
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Suggestion 1/1: Switch to '!=' check.␊
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1 | foo != bar␊
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`
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## invalid(4): !foo != bar
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> Input
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`␊
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1 | !foo != bar␊
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`
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> Error 1/1
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`␊
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> 1 | !foo != bar␊
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| ^ Negated expression in not allowed in equality check.␊
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␊
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--------------------------------------------------------------------------------␊
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Suggestion 1/1: Switch to '==' check.␊
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1 | foo == bar␊
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`
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## invalid(5): function x() { return!foo === bar; }
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> Input
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`␊
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1 | function x() {␊
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2 | return!foo === bar;␊
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3 | }␊
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`
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> Error 1/1
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`␊
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1 | function x() {␊
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> 2 | return!foo === bar;␊
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| ^ Negated expression in not allowed in equality check.␊
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3 | }␊
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␊
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--------------------------------------------------------------------------------␊
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Suggestion 1/1: Switch to '!==' check.␊
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1 | function x() {␊
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2 | return foo !== bar;␊
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3 | }␊
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`
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## invalid(6): function x() { return! foo === bar; throw! foo === bar; }
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> Input
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`␊
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1 | function x() {␊
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2 | return!␊
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3 | foo === bar;␊
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4 | throw!␊
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5 | foo === bar;␊
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6 | }␊
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`
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> Error 1/2
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`␊
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1 | function x() {␊
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> 2 | return!␊
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| ^ Negated expression in not allowed in equality check.␊
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3 | foo === bar;␊
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4 | throw!␊
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5 | foo === bar;␊
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6 | }␊
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␊
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--------------------------------------------------------------------------------␊
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Suggestion 1/1: Switch to '!==' check.␊
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1 | function x() {␊
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2 | return (␊
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3 | foo !== bar);␊
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4 | throw!␊
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5 | foo === bar;␊
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6 | }␊
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`
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> Error 2/2
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`␊
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1 | function x() {␊
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2 | return!␊
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3 | foo === bar;␊
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> 4 | throw!␊
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| ^ Negated expression in not allowed in equality check.␊
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5 | foo === bar;␊
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6 | }␊
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␊
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--------------------------------------------------------------------------------␊
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Suggestion 1/1: Switch to '!==' check.␊
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1 | function x() {␊
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2 | return!␊
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3 | foo === bar;␊
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4 | throw (␊
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5 | foo !== bar);␊
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6 | }␊
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`
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## invalid(7): foo !(a) === b
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> Input
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`␊
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1 | foo␊
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2 | !(a) === b␊
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`
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> Error 1/1
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`␊
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1 | foo␊
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> 2 | !(a) === b␊
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| ^ Negated expression in not allowed in equality check.␊
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␊
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--------------------------------------------------------------------------------␊
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Suggestion 1/1: Switch to '!==' check.␊
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1 | foo␊
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2 | ;(a) !== b␊
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`
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## invalid(8): foo ![a, b].join('') === c
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> Input
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`␊
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1 | foo␊
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2 | ![a, b].join('') === c␊
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`
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> Error 1/1
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`␊
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1 | foo␊
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> 2 | ![a, b].join('') === c␊
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| ^ Negated expression in not allowed in equality check.␊
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␊
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--------------------------------------------------------------------------------␊
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Suggestion 1/1: Switch to '!==' check.␊
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1 | foo␊
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2 | ;[a, b].join('') !== c␊
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`
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## invalid(9): foo ! [a, b].join('') === c
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> Input
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`␊
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1 | foo␊
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2 | ! [a, b].join('') === c␊
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`
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> Error 1/1
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`␊
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1 | foo␊
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> 2 | ! [a, b].join('') === c␊
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| ^ Negated expression in not allowed in equality check.␊
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␊
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--------------------------------------------------------------------------------␊
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Suggestion 1/1: Switch to '!==' check.␊
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1 | foo␊
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2 | ; [a, b].join('') !== c␊
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`
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## invalid(10): foo !/* comment */[a, b].join('') === c
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> Input
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`␊
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1 | foo␊
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2 | !/* comment */[a, b].join('') === c␊
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`
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> Error 1/1
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`␊
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1 | foo␊
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> 2 | !/* comment */[a, b].join('') === c␊
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| ^ Negated expression in not allowed in equality check.␊
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␊
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--------------------------------------------------------------------------------␊
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Suggestion 1/1: Switch to '!==' check.␊
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1 | foo␊
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2 | ;/* comment */[a, b].join('') !== c␊
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`
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## invalid(11): !!foo === bar
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> Input
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`␊
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1 | !!foo === bar␊
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`
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> Error 1/1
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`␊
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> 1 | !!foo === bar␊
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| ^ Negated expression in not allowed in equality check.␊
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␊
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--------------------------------------------------------------------------------␊
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Suggestion 1/1: Switch to '!==' check.␊
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1 | !foo !== bar␊
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`

‎test/snapshots/no-negation-in-equality-check.mjs.snap

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