jest-typescript

What this skill does

# Jest + TypeScript - Industry Standard Testing

## Overview

Jest is the industry-standard testing framework with 70% market share, providing a mature, battle-tested ecosystem for TypeScript projects. It offers comprehensive testing capabilities with built-in snapshot testing, mocking, and coverage reporting.

**Key Features**:
- 🏆 **Industry Standard**: 70% market share, widely adopted
- 📦 **All-in-One**: Test runner, assertions, mocks, coverage in one package
- 📸 **Snapshot Testing**: Built-in snapshot support for UI testing
- 🧪 **React Integration**: React Testing Library, enzyme compatibility
- 🔧 **Mature Ecosystem**: Extensive plugins, tooling, and community support
- 🎯 **TypeScript Support**: Full type safety via ts-jest
- 🔍 **Coverage Reports**: Built-in Istanbul coverage
- 🌐 **Multi-Platform**: Node.js, browser (jsdom), React Native

**Installation**:
```bash
npm install -D jest @types/jest ts-jest
npm install -D @testing-library/react @testing-library/jest-dom  # For React
```

## Basic Setup

### 1. Initialize Jest Configuration

```bash
npx ts-jest config:init
```

This creates **jest.config.js**:
```javascript
module.exports = {
  preset: 'ts-jest',
  testEnvironment: 'node',
};
```

### 2. Manual Configuration

**jest.config.ts** (TypeScript config):
```typescript
import type { Config } from 'jest';

const config: Config = {
  preset: 'ts-jest',
  testEnvironment: 'node',
  roots: ['<rootDir>/src'],
  testMatch: ['**/__tests__/**/*.ts', '**/?(*.)+(spec|test).ts'],
  moduleFileExtensions: ['ts', 'tsx', 'js', 'jsx', 'json'],
  collectCoverageFrom: [
    'src/**/*.{ts,tsx}',
    '!src/**/*.d.ts',
    '!src/**/*.test.{ts,tsx}',
    '!src/**/__tests__/**',
  ],
  coverageThreshold: {
    global: {
      branches: 80,
      functions: 80,
      lines: 80,
      statements: 80,
    },
  },
};

export default config;
```

### 3. TypeScript Configuration

**tsconfig.json**:
```json
{
  "compilerOptions": {
    "types": ["jest", "@testing-library/jest-dom"],
    "esModuleInterop": true
  }
}
```

**tsconfig.test.json** (test-specific):
```json
{
  "extends": "./tsconfig.json",
  "compilerOptions": {
    "types": ["jest", "node", "@testing-library/jest-dom"]
  },
  "include": ["src/**/*.test.ts", "src/**/*.spec.ts", "src/**/__tests__/**"]
}
```

### 4. Package.json Scripts

```json
{
  "scripts": {
    "test": "jest",
    "test:watch": "jest --watch",
    "test:coverage": "jest --coverage",
    "test:ci": "jest --ci --coverage --maxWorkers=2"
  }
}
```

## Core Testing Patterns

### Basic Test Structure

```typescript
import { describe, it, expect, beforeEach, afterEach } from '@jest/globals';

describe('Calculator', () => {
  let calculator: Calculator;

  beforeEach(() => {
    calculator = new Calculator();
  });

  afterEach(() => {
    // Cleanup
  });

  it('adds two numbers correctly', () => {
    const result = calculator.add(2, 3);
    expect(result).toBe(5);
  });

  it('handles negative numbers', () => {
    expect(calculator.add(-5, 3)).toBe(-2);
  });

  it.each([
    [1, 1, 2],
    [2, 3, 5],
    [10, -5, 5],
  ])('adds %i + %i to equal %i', (a, b, expected) => {
    expect(calculator.add(a, b)).toBe(expected);
  });
});
```

### TypeScript Type-Safe Tests

```typescript
interface User {
  id: number;
  name: string;
  email: string;
  role: 'admin' | 'user';
}

describe('User Service', () => {
  it('creates user with correct types', () => {
    const user: User = {
      id: 1,
      name: 'Alice',
      email: '[email protected]',
      role: 'admin',
    };

    // Type-safe assertions
    expect(user.id).toEqual(expect.any(Number));
    expect(user.name).toEqual(expect.any(String));
    expect(user.role).toMatch(/^(admin|user)$/);
  });

  it('validates user object shape', () => {
    const user = createUser('Bob', '[email protected]');

    expect(user).toMatchObject({
      id: expect.any(Number),
      name: 'Bob',
      email: '[email protected]',
    });
  });
});
```

## Mocking with TypeScript

### jest.mock for Module Mocking

```typescript
import { jest } from '@jest/globals';
import { UserService } from './UserService';
import * as userApi from './api/userApi';

// Mock entire module
jest.mock('./api/userApi');

describe('UserService with Mocks', () => {
  beforeEach(() => {
    jest.clearAllMocks();
  });

  it('fetches user data', async () => {
    const mockUser = { id: 1, name: 'Alice', email: '[email protected]' };

    // Type-safe mock
    const mockedFetchUser = jest.mocked(userApi.fetchUser);
    mockedFetchUser.mockResolvedValue(mockUser);

    const service = new UserService();
    const user = await service.getUser(1);

    expect(mockedFetchUser).toHaveBeenCalledWith(1);
    expect(user).toEqual(mockUser);
  });
});
```

### jest.spyOn for Method Spying

```typescript
import { jest } from '@jest/globals';

class Logger {
  log(message: string): void {
    console.log(message);
  }

  error(message: string): void {
    console.error(message);
  }
}

describe('Logger Spy', () => {
  let logger: Logger;
  let logSpy: jest.SpyInstance;

  beforeEach(() => {
    logger = new Logger();
    logSpy = jest.spyOn(logger, 'log');
  });

  afterEach(() => {
    logSpy.mockRestore();
  });

  it('tracks method calls', () => {
    logger.log('Hello');
    logger.log('World');

    expect(logSpy).toHaveBeenCalledTimes(2);
    expect(logSpy).toHaveBeenCalledWith('Hello');
    expect(logSpy).toHaveBeenLastCalledWith('World');
  });

  it('provides custom implementation', () => {
    logSpy.mockImplementation((msg: string) => {
      console.log(`[CUSTOM] ${msg}`);
    });

    logger.log('Test');
    expect(logSpy).toHaveBeenCalledWith('Test');
  });
});
```

### Type-Safe Mock Functions

```typescript
import { jest } from '@jest/globals';

interface ApiResponse<T> {
  data: T;
  status: number;
}

type FetchUserFn = (id: number) => Promise<ApiResponse<User>>;

describe('Type-Safe Mocks', () => {
  it('creates typed mock function', async () => {
    const mockFetchUser = jest.fn<FetchUserFn>()
      .mockResolvedValue({
        data: { id: 1, name: 'Alice', email: '[email protected]', role: 'user' },
        status: 200,
      });

    const result = await mockFetchUser(1);

    expect(result.data.name).toBe('Alice');
    expect(result.status).toBe(200);
    expect(mockFetchUser).toHaveBeenCalledWith(1);
  });

  it('uses mock implementation', () => {
    const mockCalculate = jest.fn<(x: number, y: number) => number>()
      .mockImplementation((x, y) => x + y);

    expect(mockCalculate(5, 3)).toBe(8);
    expect(mockCalculate).toHaveBeenCalledWith(5, 3);
  });
});
```

### Mocking Timers

```typescript
import { jest } from '@jest/globals';

describe('Timer Mocking', () => {
  beforeEach(() => {
    jest.useFakeTimers();
  });

  afterEach(() => {
    jest.useRealTimers();
  });

  it('fast-forwards time', () => {
    const callback = jest.fn();
    setTimeout(callback, 1000);

    jest.advanceTimersByTime(500);
    expect(callback).not.toHaveBeenCalled();

    jest.advanceTimersByTime(500);
    expect(callback).toHaveBeenCalledTimes(1);
  });

  it('runs all timers', () => {
    const callback = jest.fn();
    setTimeout(callback, 1000);
    setTimeout(callback, 2000);

    jest.runAllTimers();
    expect(callback).toHaveBeenCalledTimes(2);
  });

  it('handles intervals', () => {
    const callback = jest.fn();
    setInterval(callback, 1000);

    jest.advanceTimersByTime(3500);
    expect(callback).toHaveBeenCalledTimes(3);

    jest.clearAllTimers();
  });
});
```

## React Testing Library + TypeScript

### Setup for React

```bash
npm install -D @testing-library/react @testing-library/jest-dom @testing-library/user-event
npm install -D jest-environment-jsdom
```

**jest.config.ts** (React):
```typescript
import type { Config } from 'jest';

const config: Config = {
  preset: 'ts-jest',
  testEnvironment: 'jsdom',
  setupFilesAfterEnv: ['<rootDir>/src/test/setup.ts'],
  moduleNameMapper: {
    '\\.(css|less|sc