Jest Skill

Jest Testing Skill

Core Patterns

Basic Test

describe('Calculator', () => {
  let calc;
  beforeEach(() => { calc = new Calculator(); });

  test('adds two numbers', () => {
    expect(calc.add(2, 3)).toBe(5);
  });

  test('throws on division by zero', () => {
    expect(() => calc.divide(10, 0)).toThrow('Division by zero');
  });
});

Matchers

expect(value).toBe(exact);                 // === strict
expect(value).toEqual(object);             // deep equality
expect(value).toBeTruthy();
expect(value).toBeNull();
expect(value).toBeGreaterThan(3);
expect(value).toBeCloseTo(0.3, 5);
expect(str).toMatch(/regex/);
expect(arr).toContain(item);
expect(arr).toHaveLength(3);
expect(obj).toHaveProperty('name');
expect(obj).toMatchObject({ name: 'Alice' });
expect(() => fn()).toThrow(CustomError);

Mocking

// Mock function
const mockFn = jest.fn();
mockFn.mockReturnValue(42);
mockFn.mockResolvedValue({ data: 'test' });
expect(mockFn).toHaveBeenCalledWith('arg1');
expect(mockFn).toHaveBeenCalledTimes(1);

// Mock module
jest.mock('./database');
const db = require('./database');
db.getUser.mockResolvedValue({ name: 'Alice' });

// Mock with implementation
jest.mock('./api', () => ({
  fetchUsers: jest.fn().mockResolvedValue([{ name: 'Alice' }]),
}));

// Spy
const spy = jest.spyOn(console, 'log').mockImplementation();
expect(spy).toHaveBeenCalledWith('expected');
spy.mockRestore();

// Fake timers
jest.useFakeTimers();
jest.advanceTimersByTime(1000);
jest.useRealTimers();

Async Testing

test('fetches users', async () => {
  const users = await fetchUsers();
  expect(users).toHaveLength(3);
});

test('resolves with data', () => {
  return expect(fetchData()).resolves.toEqual({ data: 'value' });
});

test('rejects with error', () => {
  return expect(fetchBadData()).rejects.toThrow('not found');
});

React Component Testing (Testing Library)

import { render, screen, fireEvent, waitFor } from '@testing-library/react';
import '@testing-library/jest-dom';
import LoginForm from './LoginForm';

test('submits login form', async () => {
  const onSubmit = jest.fn();
  render();

  fireEvent.change(screen.getByLabelText('Email'), {
    target: { value: 'user@test.com' },
  });
  fireEvent.change(screen.getByLabelText('Password'), {
    target: { value: 'password123' },
  });
  fireEvent.click(screen.getByRole('button', { name: /login/i }));

  await waitFor(() => {
    expect(onSubmit).toHaveBeenCalledWith({
      email: 'user@test.com', password: 'password123',
    });
  });
});

Snapshot Testing

test('renders correctly', () => {
  const tree = renderer.create().toJSON();
  expect(tree).toMatchSnapshot();
});
// Update: jest --updateSnapshot

Anti-Patterns

| Bad | Good | Why | |-----|------|-----| | expect(x === y).toBe(true) | expect(x).toBe(y) | Better errors | | No await on async | Always await | Swallows failures | | Snapshot everything | Snapshot UI, assert logic | Snapshot fatigue |

Quick Reference

| Task | Command | |------|---------| | Run all | npx jest | | Watch | npx jest --watch | | Coverage | npx jest --coverage | | Update snapshots | npx jest --updateSnapshot | | Run file | npx jest tests/calc.test.js | | Single test | test.only('name', () => {}) |

Deep Patterns

For production-grade patterns, see reference/playbook.md:

| Section | What's Inside | |---------|--------------| | §1 Production Config | Node + React configs, path aliases, coverage thresholds | | §2 Mocking Deep Dive | Module/partial/manual mocks, spies, timers, env vars | | §3 Async Patterns | Promises, rejections, event emitters, streams | | §4 test.each | Array, tagged template, describe.each for table-driven tests | | §5 Custom Matchers | toBeWithinRange, toBeValidEmail, TypeScript declarations | | §6 React Testing Library | userEvent, hooks, context providers | | §7 Snapshot Testing | Component, inline, property matchers | | §8 API Service Testing | Mocked axios, CRUD patterns, error handling | | §9 Global Setup | Multi-project config, DB setup/teardown | | §10 CI/CD | GitHub Actions with coverage gates | | §11 Debugging Table | 10 common problems with fixes | | §12 Best Practices | 15-item production checklist |

Source & license

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

• Author: LambdaTest
• Source: LambdaTest/agent-skills
• License: MIT
• Homepage: https://agentskillsforall.com/

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