Jest Docs

Jest Skill

> Jest — A delightful JavaScript testing framework with a focus on simplicity. > Version: Jest 30.x | Docs: jestjs.io

Quick Reference

| Topic | File | Sections | |-------|------|----------| | Getting Started & Core Concepts | getting-started.md | Installation, first test, CLI usage, configuration (Babel, TypeScript, bundlers, ESLint), matchers, async testing, setup/teardown, mock functions | | API Reference | api.md | Globals (test, describe, before/after hooks, .each, .concurrent, .failing, .todo), expect matchers, jest object (mock modules, mock functions, fake timers, misc), mock function API, CLI options, configuration options | | Guides & Advanced | guides.md | Snapshot testing, timer mocks, manual mocks, ECMAScript modules, webpack integration, watch plugins, migration guide, testing frameworks (React/Vue/Angular/Express/Next.js), Puppeteer, MongoDB, DynamoDB, architecture |

Core Concepts

• Test Runner: Jest runs tests in parallel with isolated environments for speed and reliability
• Assertions: Use expect() with matchers like toBe(), toEqual(), toContain(), toThrow()
• Mock Functions: jest.fn() creates mock functions to test code links, capture calls, and configure return values
• Mock Modules: jest.mock() replaces module dependencies with mocks for isolated testing
• Snapshot Testing: Captures serialized output and compares against a reference snapshot file
• Fake Timers: jest.useFakeTimers() replaces native timer functions for controlled time advancement
• Code Coverage: Built-in coverage reporting via --coverage flag with Istanbul/V8 providers
• Watch Mode: --watch re-runs only tests related to changed files for fast development feedback
• Configuration: jest.config.js, package.json "jest" key, or CLI flags; supports projects, transforms, moduleNameMapper
• Test Environment: node (default) or jsdom (browser simulation) or custom environments
• Parallel Execution: Worker threads/processes with --maxWorkers for optimized CI performance

Jest Ecosystem

| Library | Description | |---------|-------------| | @jest/globals | Explicit imports for Jest globals (describe, expect, test) | | babel-jest | Babel integration for transpiling test files | | ts-jest | TypeScript preprocessor with source map support | | @testing-library/react | React testing utilities with Jest | | jest-puppeteer | Jest preset for E2E testing with Puppeteer | | jest-environment-jsdom | Browser-like DOM environment for Jest | | eslint-plugin-jest | ESLint plugin with Jest-specific rules | | identity-obj-proxy | Mock CSS modules in Jest | | Vitest | Vite-native alternative with Jest-compatible API |

Common Patterns

// Basic test
test('adds 1 + 2 to equal 3', () => {
  expect(sum(1, 2)).toBe(3);
});

// Grouping with describe
describe('User service', () => {
  beforeEach(() => setupDatabase());

  test('creates user', async () => {
    const user = await createUser({ name: 'Alice' });
    expect(user).toHaveProperty('id');
    expect(user.name).toBe('Alice');
  });
});

// Mocking modules
jest.mock('axios');
test('fetches data', async () => {
  axios.get.mockResolvedValue({ data: { name: 'Bob' } });
  const result = await fetchData();
  expect(result.name).toBe('Bob');
});

// Snapshot testing
it('renders correctly', () => {
  const { container } = render(Link);
  expect(container.firstChild).toMatchSnapshot();
});

Sources

• Official Jest documentation: https://jestjs.io/docs/getting-started
• Jest API reference: https://jestjs.io/docs/api
• Jest configuration: https://jestjs.io/docs/configuration
• Jest CLI options: https://jestjs.io/docs/cli
• Jest GitHub: https://github.com/jestjs/jest

Source & license

This open-source skill is cataloged on AgentStack and links to its original source — we do not rehost the code.

• Author: pledgeandgrow
• Source: pledgeandgrow/pledge-skills
• License: MIT

Install and usage instructions live in the source repository linked above.