Unit Testing with Jest: Writing Reliable JavaScript Tests

Jest is a delightful JavaScript testing framework with zero config. It handles assertions, mocking, and code coverage out of the box.

## Setup

```bash
npm install --save-dev jest
```

## Basic Test

```javascript
// sum.js
function sum(a, b) { return a + b; }
module.exports = sum;

// sum.test.js
const sum = require('./sum');

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

## Matchers

```javascript
test('object assignment', () => {
 const data = { one: 1 };
 data['two'] = 2;
 expect(data).toEqual({ one: 1, two: 2 });
});

test('null', () => {
 const n = null;
 expect(n).toBeNull();
 expect(n).toBeDefined();
 expect(n).not.toBeUndefined();
});

test('numbers', () => {
 const val = 2 + 2;
 expect(val).toBeGreaterThan(3);
 expect(val).toBeLessThan(5);
});
```

## Testing Async Code

```javascript
test('fetches data', async () => {
 const data = await fetchData();
 expect(data).toEqual({ name: 'John' });
});

// Using resolves/rejects
expect(fetchData()).resolves.toEqual({ name: 'John' });
```

## Mocking

```javascript
test('mock function', () => {
 const mockFn = jest.fn(x => x * 2);
 mockFn(5);
 expect(mockFn).toHaveBeenCalledWith(5);
 expect(mockFn).toHaveReturnedWith(10);
});
```

## Setup/Teardown

```javascript
beforeEach(() => initDatabase());
afterEach(() => clearDatabase());
beforeAll(() => startServer());
afterAll(() => stopServer());
```

## Coverage

```bash
npx jest --coverage
```

## Conclusion

Tests catch bugs early and serve as documentation. Aim for high coverage on critical paths, not 100% on everything.