eslint-plugin-performance-rules

ESLint plugin to detect performance anti-patterns in JavaScript and TypeScript codebases. Identifies code patterns that cause runtime performance degradation through static AST analysis.

Features

• 🔍 Detects quadratic loop complexity (O(n²) patterns)
• 🚫 Identifies synchronous blocking calls in loops
• ⚡ Catches expensive JSON parsing in loops
• ⚛️ Finds React inline object creation causing re-renders
• 🎯 Detects unnecessary array cloning
• ⏳ Detects await inside loops (use Promise.all instead)
• 🔤 Catches regex compilation inside loops
• 🌐 Detects DOM queries inside loops
• 🗺️ Flags O(n) array lookups inside loops (prefer Map/Set)
• 📦 Detects object spread inside loops
• 🖥️ Disallows console calls in production code
• 🔧 Works with JavaScript and TypeScript
• ✅ Zero runtime dependencies

Installation

npm install eslint-plugin-performance-rules --save-dev

Configuration

ESLint v9+ (Flat Config — eslint.config.js)

const perfPlugin = require('eslint-plugin-performance-rules');

module.exports = [
  perfPlugin.configs.recommended,
];

Or configure rules individually:

const perfPlugin = require('eslint-plugin-performance-rules');

module.exports = [
  {
    plugins: { 'performance-rules': perfPlugin },
    rules: {
      'performance-rules/no-quadratic-loops': 'warn',
      'performance-rules/no-await-in-loop': 'warn',
      'performance-rules/no-regex-in-loop': 'warn',
      'performance-rules/no-console-in-production': 'warn',
      // ...add more as needed
    },
  },
];

ESLint v7/v8 (Legacy Config — .eslintrc.js)

module.exports = {
  plugins: ['performance-rules'],
  extends: ['plugin:performance-rules/recommended-legacy'],
};

Or configure rules individually:

module.exports = {
  plugins: ['performance-rules'],
  rules: {
    'performance-rules/no-quadratic-loops': 'warn',
    'performance-rules/no-await-in-loop': 'warn',
    'performance-rules/no-regex-in-loop': 'warn',
    'performance-rules/no-console-in-production': 'warn',
  },
};

TypeScript Projects

// eslint.config.js
const perfPlugin = require('eslint-plugin-performance-rules');
const tsParser = require('@typescript-eslint/parser');

module.exports = [
  {
    files: ['**/*.ts', '**/*.tsx'],
    languageOptions: {
      parser: tsParser,
      parserOptions: { ecmaVersion: 2020, sourceType: 'module' },
    },
    ...perfPlugin.configs.recommended,
  },
];

Rules

| Rule | Description | Default | |------|-------------|---------| | no-quadratic-loops | Detects nested loops over the same array — O(n²) complexity | warn | | no-sync-in-loop | Detects fs.*Sync / child_process.*Sync calls inside loops | warn | | no-large-json-parse-in-loop | Detects JSON.parse() and JSON.stringify() inside loops | warn | | react-no-inline-object-creation | Detects inline objects/arrays/functions in JSX props | warn | | no-unnecessary-array-clone | Detects array clones that are never mutated | warn | | no-await-in-loop | Detects await inside loops — use Promise.all() instead | warn | | no-regex-in-loop | Detects regex literals / new RegExp() inside loops | warn | | no-dom-query-in-loop | Detects document.querySelector and friends inside loops | warn | | no-linear-search-in-loop | Flags O(n) array lookups (find/filter/includes/indexOf/findIndex) inside loops | warn | | no-object-spread-in-loop | Detects {...obj} / Object.assign({}, ...) inside loops | warn | | no-console-in-production | Disallows console.* calls (except error/warn by default) | warn |

Rule Details

no-quadratic-loops

// Bad
for (const a of arr) {
  for (const b of arr) { } // O(n²)
}

// Good
const set = new Set(arr);
for (const a of arr) { set.has(a); }

no-await-in-loop

// Bad
for (const url of urls) {
  await fetch(url); // sequential
}

// Good
await Promise.all(urls.map(url => fetch(url)));

no-regex-in-loop

// Bad
for (const s of strings) {
  /foo/.test(s); // recompiled every iteration
}

// Good
const re = /foo/;
for (const s of strings) { re.test(s); }

no-dom-query-in-loop

// Bad
for (const item of items) {
  document.querySelector('.list').appendChild(item);
}

// Good
const list = document.querySelector('.list');
for (const item of items) { list.appendChild(item); }

no-linear-search-in-loop

// Bad — O(n²): find/filter/includes/indexOf/findIndex inside a loop
for (const id of ids) {
  users.find(u => u.id === id);
}

// Good — O(1) lookup with a Map built once outside the loop
const userMap = new Map(users.map(u => [u.id, u]));
for (const id of ids) { userMap.get(id); }

no-object-spread-in-loop

// Bad — new object every iteration
for (const item of arr) {
  const copy = { ...item };
}

// Good
const target = {};
for (const item of arr) { Object.assign(target, item); }

no-console-in-production

// Bad
console.log('debug value:', x);

// Good
console.error('something broke'); // error/warn allowed by default

Configure allowed methods:

// eslint.config.js
rules: {
  'performance-rules/no-console-in-production': ['warn', { allow: ['error', 'warn', 'info'] }]
}

Usage

# Run ESLint
npx eslint .

# With performance formatter
npx eslint . --format ./node_modules/eslint-plugin-performance-rules/src/lib/formatters/performance.js

License

MIT