Jest Testing Skill
Core Patterns
Basic Test
javascript
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
javascript
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
javascript
// 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
javascript
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)
javascript
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(<LoginForm onSubmit={onSubmit} />);
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
javascript
test('renders correctly', () => {
const tree = renderer.create(<Button label="Click" />).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 |