test-patterns

# Test Patterns

Write, run, and debug tests across languages. Covers unit tests, integration tests, E2E tests, mocking, coverage, and TDD workflows.

## When to Use

- Setting up a test suite for a new project
- Writing unit tests for functions or modules
- Writing integration tests for APIs or database interactions
- Setting up code coverage measurement
- Mocking external dependencies (APIs, databases, file system)
- Debugging flaky or failing tests
- Implementing test-driven development (TDD)

## Node.js (Jest / Vitest)

### Setup

```bash
# Jest
npm install -D jest
# Add to package.json: "scripts": { "test": "jest" }

# Vitest (faster, ESM-native)
npm install -D vitest
# Add to package.json: "scripts": { "test": "vitest" }
```

### Unit Tests

```javascript
// math.js
export function add(a, b) { return a + b; }
export function divide(a, b) {
  if (b === 0) throw new Error('Division by zero');
  return a / b;
}

// math.test.js
import { add, divide } from './math.js';

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

  test('handles negative numbers', () => {
    expect(add(-1, 1)).toBe(0);
  });

  test('handles zero', () => {
    expect(add(0, 0)).toBe(0);
  });
});

describe('divide', () => {
  test('divides two numbers', () => {
    expect(divide(10, 2)).toBe(5);
  });

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

  test('handles floating point', () => {
    expect(divide(1, 3)).toBeCloseTo(0.333, 3);
  });
});
```

### Async Tests

```javascript
// api.test.js
import { fetchUser } from './api.js';

test('fetches user by id', async () => {
  const user = await fetchUser('123');
  expect(user).toHaveProperty('id', '123');
  expect(user).toHaveProperty('name');
  expect(user.name).toBeTruthy();
});

test('throws on missing user', async () => {
  await expect(fetchUser('nonexistent')).rejects.toThrow('Not found');
});
```

### Mocking

```...