ESLint for Test Quality

ESLint for Test Quality

This skill makes an AI agent wire ESLint plugins that lint the tests themselves - catching focused tests that silently skip entire suites in CI, assertion-free tests, conditional expects, and flaky waitForTimeout calls before they merge. Trigger it when a project has test files but no test-specific lint rules, when an it.only ever reaches main, or when the user asks to "lint tests", "block .only", or "enforce testing best practices".

Core Principles

1. A committed .only is a silent CI outage. it.only makes every other test in the file stop running while the build stays green. no-focused-tests set to error is the single highest-value test lint rule; it is non-negotiable.
2. Tests without assertions must fail the lint. A test that calls code and asserts nothing passes forever. expect-expect (Jest/Playwright) turns these into lint errors and accepts custom assertion wrappers via configuration.
3. Lint rules encode review comments you are tired of writing. "Do not put expects in conditionals", "use userEvent not fireEvent", "no waitForTimeout" - each has a rule; automate the comment.
4. Scope test rules to test files only. Apply plugin configs with files: ['**/*.test.ts'] globs in flat config so production code is not subjected to test rules and vice versa.
5. Warnings are noise; errors are gates. CI must run with --max-warnings 0 or set every rule you care about to error. A warning that scrolls by in CI logs changes nothing.
6. Adopt recommended configs first, then tighten. Start from flat/recommended for each plugin, then promote the high-signal rules (no-conditional-expect, no-standalone-expect, prefer-user-event) to error as the suite cleans up.

Setup

npm install --save-dev eslint eslint-plugin-jest eslint-plugin-testing-library \
  eslint-plugin-jest-dom eslint-plugin-playwright

Flat config with per-suite scoping

// eslint.config.js
import jest from 'eslint-plugin-jest';
import testingLibrary from 'eslint-plugin-testing-library';
import jestDom from 'eslint-plugin-jest-dom';
import playwright from 'eslint-plugin-playwright';

export default [
  // Unit and component tests (Jest + Testing Library)
  {
    files: ['src/**/*.test.{ts,tsx}', 'src/**/__tests__/**/*.{ts,tsx}'],
    plugins: { jest, 'testing-library': testingLibrary, 'jest-dom': jestDom },
    languageOptions: { globals: jest.environments.globals.globals },
    rules: {
      ...jest.configs['flat/recommended'].rules,
      ...testingLibrary.configs['flat/react'].rules,
      ...jestDom.configs['flat/recommended'].rules,
      'jest/no-focused-tests': 'error',
      'jest/no-disabled-tests': 'warn',
      'jest/no-conditional-expect': 'error',
      'jest/no-standalone-expect': 'error',
      'jest/valid-title': 'error',
      'jest/prefer-hooks-on-top': 'error',
      'jest/expect-expect': [
        'error',
        { assertFunctionNames: ['expect', 'expectTypeOf', 'assertOrderShape'] },
      ],
      'testing-library/prefer-user-event': 'error',
      'testing-library/no-wait-for-side-effects': 'error',
      'testing-library/no-manual-cleanup': 'error',
    },
  },
  // Playwright E2E specs
  {
    files: ['e2e/**/*.spec.ts'],
    plugins: { playwright },
    rules: {
      ...playwright.configs['flat/recommended'].rules,
      'playwright/no-focused-test': 'error',
      'playwright/no-skipped-test': 'warn',
      'playwright/no-wait-for-timeout': 'error',
      'playwright/no-conditional-in-test': 'error',
      'playwright/no-force-option': 'error',
      'playwright/expect-expect': 'error',
      'playwright/no-networkidle': 'error',
      'playwright/prefer-web-first-assertions': 'error',
    },
  },
];