ESLint & Prettier configuration

I've been starting a lot of new mini projects lately, and I believe a properly set up linter and formatter are essential for a good development experience. Instead of reconfiguring my environment every time, I use this simple configuration to get started quickly.

import js from '@eslint/js';
import eslintConfigPrettier from 'eslint-config-prettier/flat';
import globals from './globals.js';

export default [
  js.configs.recommended,
  eslintConfigPrettier,
  {
    languageOptions: {
      ecmaVersion: 2025,
      sourceType: 'module',
      globals: { ...globals },
    },
    rules: {
      'no-duplicate-imports': 'error',
      'no-useless-computed-key': 'error',
      'no-console': 'off',
      eqeqeq: ['error', 'smart'],
      curly: 'off',
      'object-shorthand': ['warn', 'always'],
      camelcase: ['warn', { properties: 'always' }],
      'no-extend-native': 'error',
      'no-loop-func': 'error',
      'no-implied-eval': 'error',
      'no-iterator': 'error',
      'no-label-var': 'error',
      'no-multi-str': 'error',
      'no-script-url': 'error',
      'no-shadow-restricted-names': 'error',
      'no-spaced-func': 'error',
      'no-sparse-arrays': 'warn',
      'no-fallthrough': 'warn',
      'no-caller': 'error',
      'no-eval': 'error',
      'no-negated-in-lhs': 'error',
      'no-new': 'error',
      'no-new-require': 'error',
      'block-scoped-var': 'error',
      'no-use-before-define': 'warn',
      'no-proto': 'error',
      complexity: ['warn', 50],
      'new-parens': 'error',
      yoda: ['error', 'never'],
      'no-useless-assignment': 'error',
    },
  },
];

{
  "printWidth": 80,
  "arrowParens": "avoid",
  "singleQuote": true,
  "tabWidth": 2,
  "useTabs": false,
  "semi": true,
  "trailingComma": "es5",
  "bracketSpacing": true,
  "objectWrap": "collapse"
}

Package versions

This setup is optimized for ESLint & Prettier, and revised for versions 9.24.0 and 3.5.3 of eslint and prettier, respectively. I also use eslint-config-prettier for compatibility between the two tools.

Finally, you'll notice an imported globals.js file. I'm not 100% sure where I got mine, but you can grab one from this amazing GitHub repository or install the package globals from npm. This file contains a list of global variables that are available in various environments, such as Node.js, browsers, and more. It helps ESLint understand which variables are defined globally in your code.

ESLint

ESLint has a ton of rules and I can't possibly hope to cover them all myself. The defaults are pretty good, but some tweaks are necessary to make it match my taste and the setup most of the teams I work with use.

General setup

• js.configs.recommended: Use the recommended ESLint configuration.
• eslintConfigPrettier: Compatibility with Prettier.
• languageOptions:
  • ecmaVersion: Set to 2025 to use the latest JavaScript features.
  • sourceType: Set to module to use ESM.
  • globals: Import the globals from the globals.js file (see above).

Rules

• no-duplicate-imports: Disallow duplicate imports.
• no-useless-computed-key: Disallow unnecessary computed property keys.
• no-console: Allow console statements (you may want to change this in production).
• eqeqeq: Enforce strict equality checks (=== and !==).
• curly: Disabled, as I prefer to use if statements without braces when possible.
• object-shorthand: Warn if object literals are not using shorthand syntax.
• camelcase: Warn if camelCase naming convention aren't followed for object properties.
• no-extend-native: Disallow extending native objects.
• no-loop-func: Disallow function declarations that are nested inside loops.
• no-implied-eval: Disallow implied eval().
• no-iterator: Disallow the use of __iterator__ property.
• no-label-var: Disallow labels that are also variable names.
• no-multi-str: Disallow multiline strings.
• no-script-url: Disallow javascript: URLs.
• no-shadow-restricted-names: Disallow shadowing of restricted names.
• no-sparse-arrays: Warn about sparse arrays.
• no-fallthrough: Warn on fallthrough cases in switch statements.
• no-caller: Disallow the use of arguments.caller or arguments.callee.
• no-eval: Disallow the use of eval().
• no-negated-in-lhs: Disallow negation of the left operand in in expressions.
• no-new: Disallow the use of new without assignment.
• no-new-require: Disallow new for require() calls.
• block-scoped-var: Enforce block scoping for variables.
• no-use-before-define: Warn about using variables before they are defined.
• no-proto: Disallow the use of __proto__.
• complexity: Warn about complex functions.
• new-parens: Enforce parentheses for constructor calls.
• yoda: Enforce Yoda conditions (e.g., if (true === condition)).
• no-useless-assignment: Disallow unnecessary assignments.

Prettier

Prettier's setup is really straightforward:

• printWidth: I don't like long lines of code, hence the 80 character limit.
• arrowParens: I prefer to avoid parentheses in arrow function if possible.
• singleQuote: Single quotes seem less cluttered to me.
• tabWidth: Naturally, 2 spaces, for uniformity. Your mileage may vary.
• useTabs: Spaces over tabs, always.
• semi: Force semicolons at the end of statements. I may change this in the future.
• trailingComma: This is probably the most controversial one, but I find that adding trailing commas in multiline objects and arrays is friendlier to version control.
• bracketSpacing: Space inside brackets for better readability.
• objectWrap: Collapse objects to a single line if possible. It helps with longer files, but I don't feel super strongly about it.