Jest Testing Skill

---
name: jest-skill-c4027b9faea9
description: >
  Generates Jest unit and integration tests in JavaScript or TypeScript.
  Covers mocking, snapshots, async testing, and React component testing.
  Use when user mentions "Jest", "describe/it/expect", "jest.mock",
  "toMatchSnapshot". Triggers on: "Jest", "expect().toBe()", "jest.mock",
  "snapshot test", "JS test", "React test".
languages:
  - JavaScript
  - TypeScript
category: unit-testing
license: MIT
metadata:
  author: TestMu AI
  version: "1.0"
---

# 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 |