What is Jest? What are its main features and how do you write a basic test?

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Accepted Answer

## What is Jest?

Jest is a **JavaScript testing framework** developed by Facebook. It's the most popular testing framework for React applications and works great with Node.js, TypeScript, and more.

## Key Features

1. **Zero Configuration** - Works out of the box
2. **Snapshot Testing** - Captures UI snapshots
3. **Built-in Mocking** - Easy to mock functions/modules
4. **Code Coverage** - Built-in coverage reports
5. **Parallel Testing** - Fast test execution
6. **Watch Mode** - Re-runs tests on file changes

## Basic Test Structure

```javascript
// math.js
export function add(a, b) {
  return a + b;
}

export function multiply(a, b) {
  return a * b;
}
```

```javascript
// math.test.js
import { add, multiply } from './math';

describe('Math functions', () => {
  test('add returns the sum of two numbers', () => {
    expect(add(2, 3)).toBe(5);
  });

  test('multiply returns the product of two numbers', () => {
    expect(multiply(3, 4)).toBe(12);
  });
});
```

## Common Matchers

```javascript
// Equality
expect(value).toBe(5);           // Strict equality (===)
expect(value).toEqual({a: 1});   // Deep equality for objects

// Truthiness
expect(value).toBeTruthy();
expect(value).toBeFalsy();
expect(value).toBeNull();
expect(value).toBeUndefined();
expect(value).toBeDefined();

// Numbers
expect(value).toBeGreaterThan(3);
expect(value).toBeLessThanOrEqual(10);
expect(0.1 + 0.2).toBeCloseTo(0.3);  // Floating point

// Strings
expect(value).toMatch(/pattern/);

// Arrays
expect(array).toContain('item');
expect(array).toHaveLength(3);

// Exceptions
expect(() => throwingFn()).toThrow();
expect(() => throwingFn()).toThrow('error message');
```

## Setup and Teardown

```javascript
describe('Database tests', () => {
  beforeAll(() => {
    // Run once before all tests
    return initializeDatabase();
  });

  afterAll(() => {
    // Run once after all tests
    return closeDatabase();
  });

  beforeEach(() => {
    // Run before each test
    return clearDatabase();
  });

  afterEach(() => {
    // Run after each test
  });

  test('...', () => {});
});
```

## File Naming Convention

Jest automatically finds test files:
- `*.test.js` or `*.test.ts`
- `*.spec.js` or `*.spec.ts`
- Files in `__tests__` folder

## Running Tests

```bash
# Run all tests
npm test

# Run with coverage
npm test -- --coverage

# Run in watch mode
npm test -- --watch

# Run specific file
npm test -- math.test.js
```