Eslint testing

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',
    },
  },
];

Patterns

1. What the rules actually catch

// e2e/checkout.spec.ts - every line below is a lint ERROR with the config above

test.only('applies a coupon', async ({ page }) => {
  // playwright/no-focused-test: the other 84 specs in this project
  // would silently not run in CI while the build stays green.
});

test('waits for the cart to update', async ({ page }) => {
  await page.waitForTimeout(3000); // playwright/no-wait-for-timeout: flaky AND slow
  await page.click('#checkout', { force: true }); // playwright/no-force-option: bypasses actionability
});

test('shows totals', async ({ page }) => {
  const rows = await page.locator('.row').count();
  if (rows > 0) {
    expect(rows).toBeGreaterThan(0); // playwright/no-conditional-in-test
  }
  // playwright/expect-expect also fires if no assertion is reachable
});

// src/components/CouponForm.test.tsx - Jest + Testing Library violations

it('test 1', async () => {
  // jest/valid-title: meaningless title
  render(<CouponForm />);
  fireEvent.change(screen.getByRole('textbox'), { target: { value: 'SAVE10' } });
  // testing-library/prefer-user-event: fireEvent skips focus/keyboard semantics
});

it('submits the coupon', async () => {
  try {
    await submitCoupon('SAVE10');
    expect(api.apply).toHaveBeenCalled();
  } catch {
    expect(true).toBe(false); // jest/no-conditional-expect: may never run
  }
});

2. Teaching expect-expect about custom assertion helpers

// tests/helpers/assert-order-shape.ts
import { expect } from 'vitest';

// Custom assertion helper used across suites
export function assertOrderShape(order: unknown): void {
  expect(order).toMatchObject({
    id: expect.stringMatching(/^ord_/),
    total: expect.any(Number),
    items: expect.arrayContaining([expect.objectContaining({ sku: expect.any(String) })]),
  });
}
// Registered above via expect-expect assertFunctionNames so tests
// that only call assertOrderShape(...) do not trip the rule.

3. CI lint gate that blocks merges

# .github/workflows/lint.yml
name: lint
on: [pull_request]

jobs:
  eslint:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v4
      - uses: actions/setup-node@v4
        with:
          node-version: 20
          cache: npm
      - run: npm ci
      - name: Lint (zero warnings allowed)
        run: npx eslint . --max-warnings 0

4. Pre-commit guard for the fast feedback loop

npm install --save-dev husky lint-staged
npx husky init

{
  "lint-staged": {
    "*.{ts,tsx,js}": ["eslint --fix --max-warnings 0"]
  }
}

# .husky/pre-commit
npx lint-staged

Best Practices

• Run eslint --fix first when adopting rules on an existing suite; many testing-library rules (such as query preferences) auto-fix.
• Pair jest/no-disabled-tests as warn with a tracking convention: every it.skip requires a linked ticket in a comment.
• Add playwright/no-networkidle and prefer-web-first-assertions early; they remove the two most common Playwright flake sources.
• For Vitest projects, use eslint-plugin-vitest (largely rule-compatible with eslint-plugin-jest) and the same scoping strategy.
• Keep the lint job separate from the test job in CI so a lint failure reports in seconds, not after a 10-minute test run.
• Re-run npx eslint . --max-warnings 0 locally before pushing; the gate exists to be unreachable, not to be hit.

Anti-Patterns

• Putting test rules in the global config so production files get flagged for "missing expect" and developers disable the plugin entirely.
• Setting no-focused-tests to warn: the one severity that cannot stop the exact accident the rule exists for.
• Blanket // eslint-disable-next-line comments without a reason; require --report-unused-disable-directives (or linterOptions.reportUnusedDisableDirectives) to keep disables honest.
• Linting only changed files in CI while the repository still contains .only from last month - run the full lint, it is cheap.
• Treating the lint gate as optional ("we will fix warnings later"): later never arrives; gate at zero from day one on new projects.
• Writing a custom rule for something eslint-plugin-jest already ships; check the plugin's rule list first.

When to Trigger This Skill

• A repository has Jest, Vitest, Playwright, or Testing Library tests but eslint.config.js contains no test-specific plugins.
• An it.only, test.only, or fdescribe was found on the main branch, or CI passed while most tests silently did not run.
• The user asks to "lint tests", "ban waitForTimeout", "enforce userEvent", "block focused tests", or "add a lint gate to CI".
• Migrating from .eslintrc to flat config and the test-file overrides need translating into files-scoped config objects.
• Code review keeps repeating the same test-hygiene comments; convert each into the matching rule and let the linter say it.