Jest

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:

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

npx ts-jest config:init

This creates jest.config.js:

module.exports = {
  preset: 'ts-jest',
  testEnvironment: 'node',
};

2. Manual Configuration

jest.config.ts (TypeScript config):

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:

{
  "compilerOptions": {
    "types": ["jest", "@testing-library/jest-dom"],
    "esModuleInterop": true
  }
}

tsconfig.test.json (test-specific):

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

4. Package.json Scripts

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

Core Testing Patterns

Basic Test Structure

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

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: 'alice@example.com',
      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', 'bob@example.com');

    expect(user).toMatchObject({
      id: expect.any(Number),
      name: 'Bob',
      email: 'bob@example.com',
    });
  });
});

Mocking with TypeScript

jest.mock for Module Mocking

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: 'alice@example.com' };

    // 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

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

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: 'alice@example.com', 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

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

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

jest.config.ts (React):

import type { Config } from 'jest';

const config: Config = {
  preset: 'ts-jest',
  testEnvironment: 'jsdom',
  setupFilesAfterEnv: ['<rootDir>/src/test/setup.ts'],
  moduleNameMapper: {
    '\\.(css|less|scss|sass)$': 'identity-obj-proxy',
    '\\.(jpg|jpeg|png|gif|svg)$': '<rootDir>/__mocks__/fileMock.js',
  },
  transform: {
    '^.+\\.tsx?$': ['ts-jest', {
      tsconfig: {
        jsx: 'react-jsx',
      },
    }],
  },
};

export default config;

src/test/setup.ts:

import '@testing-library/jest-dom';
import { cleanup } from '@testing-library/react';
import { afterEach } from '@jest/globals';

afterEach(() => {
  cleanup();
});

React Component Testing

import { render, screen, waitFor } from '@testing-library/react';
import userEvent from '@testing-library/user-event';
import { Counter } from './Counter';

describe('Counter Component', () => {
  it('renders initial count', () => {
    render(<Counter initialCount={0} />);
    expect(screen.getByText('Count: 0')).toBeInTheDocument();
  });

  it('increments counter on button click', async () => {
    const user = userEvent.setup();
    render(<Counter initialCount={0} />);

    const button = screen.getByRole('button', { name: /increment/i });
    await user.click(button);

    expect(screen.getByText('Count: 1')).toBeInTheDocument();
  });

  it('calls onChange callback with correct value', async () => {
    const
    const user = userEvent.setup();

    render(<Counter initialCount={5} />);

    await user.click(screen.getByRole('button', { name: /increment/i }));

    expect(onChange).toHaveBeenCalledWith(6);
    expect(onChange).toHaveBeenCalledTimes(1);
  });

  it('disables button when max count reached', () => {
    render(<Counter initialCount={10} maxCount={10} />);

    const button = screen.getByRole('button', { name: /increment/i });
    expect(button).toBeDisabled();
  });
});

Testing Hooks

import { renderHook, act } from '@testing-library/react';
import { useCounter } from './useCounter';

describe('useCounter Hook', () => {
  it('initializes with default value', () => {
    const { result } = renderHook(() => useCounter(0));
    expect(result.current.count).toBe(0);
  });

  it('increments counter', () => {
    const { result } = renderHook(() => useCounter(0));

    act(() => {
      result.current.increment();
    });

    expect(result.current.count).toBe(1);
  });

  it('decrements counter', () => {
    const { result } = renderHook(() => useCounter(5));

    act(() => {
      result.current.decrement();
    });

    expect(result.current.count).toBe(4);
  });

  it('resets to initial value', () => {
    const { result } = renderHook(() => useCounter(10));

    act(() => {
      result.current.increment();
      result.current.increment();
    });

    expect(result.current.count).toBe(12);

    act(() => {
      result.current.reset();
    });

    expect(result.current.count).toBe(10);
  });
});

Testing Async Components

import { render, screen, waitFor } from '@testing-library/react';
import userEvent from '@testing-library/user-event';
import { UserProfile } from './UserProfile';
import * as api from './api';

jest.mock('./api');

describe('UserProfile Async', () => {
  it('loads and displays user data', async () => {
    const mockUser = { id: 1, name: 'Alice', email: 'alice@example.com' };
    jest.mocked(api.fetchUser).mockResolvedValue(mockUser);

    render(<UserProfile userId={1} />);

    expect(screen.getByText('Loading...')).toBeInTheDocument();

    await waitFor(() => {
      expect(screen.getByText('Alice')).toBeInTheDocument();
    });

    expect(screen.getByText('alice@example.com')).toBeInTheDocument();
  });

  it('displays error on fetch failure', async () => {
    jest.mocked(api.fetchUser).mockRejectedValue(new Error('Network error'));

    render(<UserProfile userId={1} />);

    await waitFor(() => {
      expect(screen.getByText(/error/i)).toBeInTheDocument();
    });
  });
});

Snapshot Testing

Component Snapshots

import { render } from '@testing-library/react';
import { UserCard } from './UserCard';

describe('UserCard Snapshots', () => {
  it('matches snapshot for regular user', () => {
    const { container } = render(
      <UserCard
        name="Alice"
        email="alice@example.com"
        role="user"
      />
    );

    expect(container.firstChild).toMatchSnapshot();
  });

  it('matches snapshot for admin user', () => {
    const { container } = render(
      <UserCard
        name="Bob"
        email="bob@example.com"
        role="admin"
      />
    );

    expect(container.firstChild).toMatchSnapshot();
  });

  it('uses inline snapshot', () => {
    const user = { id: 1, name: 'Charlie', role: 'user' };

    expect(user).toMatchInlineSnapshot(`
      {
        "id": 1,
        "name": "Charlie",
        "role": "user",
      }
    `);
  });
});

Updating Snapshots

# Update all snapshots
jest --updateSnapshot
jest -u

# Update snapshots for specific test file
jest UserCard.test.tsx -u

# Interactive snapshot update
jest --watch
# Press 'u' to update failing snapshots

Custom Snapshot Serializers

// __tests__/serializers/dateSerializer.ts
export default {
  test: (val: any) => val instanceof Date,
  print: (val: Date) => `Date(${val.toISOString()})`,
};

jest.config.ts:

const config: Config = {
  snapshotSerializers: ['<rootDir>/__tests__/serializers/dateSerializer.ts'],
};

Async Testing

Testing Promises

import { fetchData, saveData } from './api';

describe('Async Operations', () => {
  it('resolves with data', async () => {
    const data = await fetchData(1);
    expect(data).toBeDefined();
    expect(data.id).toBe(1);
  });

  it('handles promise rejection', async () => {
    await expect(fetchData(-1)).rejects.toThrow('Invalid ID');
  });

  it('uses resolves matcher', async () => {
    await expect(fetchData(1)).resolves.toHaveProperty('id', 1);
  });

  it('tests multiple async operations', async () => {
    const [user, posts] = await Promise.all([
      fetchUser(1),
      fetchPosts(1),
    ]);

    expect(user.id).toBe(1);
    expect(posts).toHaveLength(expect.any(Number));
  });
});

Testing Callbacks

describe('Callback Testing', () => {
  it('calls callback with correct arguments', (done) => {
    function fetchWithCallback(id: number, callback: (data: any) => void) {
      setTimeout(() => {
        callback({ id, name: 'Test' });
      }, 100);
    }

    fetchWithCallback(1, (data) => {
      try {
        expect(data.id).toBe(1);
        expect(data.name).toBe('Test');
        done();
      } catch (error) {
        done(error);
      }
    });
  });
});

Coverage Configuration

Advanced Coverage Setup

jest.config.ts:

const config: Config = {
  collectCoverage: true,
  coverageDirectory: 'coverage',
  coverageProvider: 'v8', // or 'babel' for compatibility
  coverageReporters: ['text', 'lcov', 'html', 'json'],
  collectCoverageFrom: [
    'src/**/*.{ts,tsx}',
    '!src/**/*.d.ts',
    '!src/**/*.test.{ts,tsx}',
    '!src/**/__tests__/**',
    '!src/index.ts',
    '!src/types/**',
  ],
  coverageThreshold: {
    global: {
      branches: 80,
      functions: 80,
      lines: 80,
      statements: 80,
    },
    './src/core/': {
      branches: 90,
      functions: 90,
      lines: 90,
      statements: 90,
    },
  },
  coveragePathIgnorePatterns: [
    '/node_modules/',
    '/dist/',
    '/__tests__/',
  ],
};

Running Coverage

# Generate coverage report
npm test -- --coverage

# Coverage with watch mode
npm test -- --coverage --watch

# Coverage for specific files
npm test -- --coverage --collectCoverageFrom="src/components/**/*.tsx"

# View HTML report
open coverage/lcov-report/index.html

Migration from Vitest

Key Differences

API Changes:

// Vitest
import { vi } from 'vitest';
const mockFn = vi.fn();
vi.spyOn(obj, 'method');

// Jest
import { jest } from '@jest/globals';
const mockFn = jest.fn();
jest.spyOn(obj, 'method');

Migration Checklist

1. Update Dependencies:

npm uninstall vitest @vitest/ui
npm install -D jest @types/jest ts-jest

2. Update package.json:

{
  "scripts": {
    "test": "jest",           // Was: vitest run
    "test:watch": "jest --watch"  // Was: vitest
  }
}

3. Replace vitest.config.ts with jest.config.ts:

// Old: vitest.config.ts
import { defineConfig } from 'vitest/config';
export default defineConfig({
  test: {
    globals: true,
    environment: 'jsdom',
  },
});

// New: jest.config.ts
import type { Config } from 'jest';
const config: Config = {
  preset: 'ts-jest',
  testEnvironment: 'jsdom',
  globals: {
    'ts-jest': {
      isolatedModules: true,
    },
  },
};
export default config;

4. Update Test Files:

// Change imports
- import { vi } from 'vitest';
+ import { jest } from '@jest/globals';

// Update mocks
- vi.fn()
+ jest.fn()

- vi.spyOn()
+ jest.spyOn()

- vi.mock()
+ jest.mock()

// Timer mocks
- vi.useFakeTimers()
+ jest.useFakeTimers()

- vi.advanceTimersByTime()
+ jest.advanceTimersByTime()

5. Update tsconfig.json:

{
  "compilerOptions": {
    "types": ["jest", "@testing-library/jest-dom"]  // Was: vitest/globals
  }
}

Jest vs Vitest Comparison

Performance

Jest:

• Slower initial startup (no HMR)
• Sequential test execution by default
• 1-5 seconds for medium projects

Vitest:

• Instant HMR-based execution
• Parallel by default
• 100-500ms for same projects

Ecosystem

Jest:

• ✅ 70% market share
• ✅ Mature ecosystem (8+ years)
• ✅ More Stack Overflow answers
• ✅ Better corporate support

Vitest:

• ✅ Modern, growing adoption
• ✅ Vite-native integration
• ⚠️ Smaller ecosystem
• ⚠️ Fewer resources

TypeScript Support

Jest:

• Requires ts-jest configuration
• Extra transform step
• Slower compilation

Vitest:

• Built-in TypeScript support
• No configuration needed
• Faster through Vite

When to Use Jest

Choose Jest for:

• ✅ Existing projects already using Jest
• ✅ Corporate environments requiring proven tools
• ✅ Projects requiring extensive ecosystem support
• ✅ React projects with Create React App
• ✅ Non-Vite build systems (Webpack, Rollup)

Choose Vitest for:

• ✅ New projects with modern tooling
• ✅ Vite-based applications
• ✅ Performance-critical test suites
• ✅ ESM-first projects

Best Practices

1. Use TypeScript Configuration: Type-safe tests prevent runtime errors
2. Mock External Dependencies: Network, file system, databases
3. Isolate Tests: Each test should be independent
4. Use describe Blocks: Group related tests logically
5. Clear Mock State: Use jest.clearAllMocks() in beforeEach
6. Test Edge Cases: Empty arrays, null, undefined, errors
7. Use .each for Data-Driven Tests: Test multiple inputs efficiently
8. Avoid Testing Implementation: Test behavior, not internal structure
9. Keep Tests Fast: Mock slow operations, use parallel execution
10. Maintain Coverage Thresholds: Enforce minimum coverage in CI

Common Pitfalls

❌ Not clearing mocks between tests:

// WRONG - mocks leak between tests
it('test 1', () => {
  jest.spyOn(api, 'fetch');
  // No cleanup!
});

// CORRECT
afterEach(() => {
  jest.restoreAllMocks();
});

❌ Forgetting to await async tests:

// WRONG - test completes before assertion
it('fetches data', () => {
  fetchData().then(data => {
    expect(data).toBeDefined();  // Never runs!
  });
});

// CORRECT
it('fetches data', async () => {
  const data = await fetchData();
  expect(data).toBeDefined();
});

❌ Using wrong test environment:

// WRONG - testing DOM without jsdom
// jest.config.ts
testEnvironment: 'node',  // Can't test React!

// CORRECT
testEnvironment: 'jsdom',

❌ Not using TypeScript types for mocks:

// WRONG - no type safety
const mockFn = jest.fn();

// CORRECT
const mockFn = jest.fn<(id: number) => Promise<User>>();

Resources

• Documentation: https://jestjs.io/docs/getting-started
• TypeScript Guide: https://jestjs.io/docs/getting-started#using-typescript
• ts-jest: https://kulshekhar.github.io/ts-jest/
• React Testing Library: https://testing-library.com/docs/react-testing-library/intro/
• Jest DOM Matchers: https://github.com/testing-library/jest-dom

Related Skills

When using Jest, consider these complementary skills:

• typescript-core: Advanced TypeScript patterns, tsconfig optimization, and type safety
• react: React component testing patterns with Testing Library
• vitest: Modern alternative with Vite-native performance and faster execution

Quick TypeScript Type Safety Reference (Inlined for Standalone Use)

// Type-safe test helpers with generics
function createMockUser<T extends Partial<User>>(overrides: T): User & T {
  return {
    id: 1,
    name: 'Test User',
    email: 'test@example.com',
    ...overrides
  };
}

// Usage with type inference
const adminUser = createMockUser({ role: 'admin' });
// Type: User & { role: string }

// Type-safe mock functions
const mockFetch = jest.fn<typeof fetch>();
mockFetch.mockResolvedValue(new Response('{}'));

// Const type parameters for literal types
const createConfig = <const T extends Record<string, unknown>>(config: T): T => config;
const testConfig = createConfig({ environment: 'test', debug: true });
// Type: { environment: "test"; debug: true } (literals preserved)

Quick React Testing Patterns (Inlined for Standalone Use)

// React Testing Library with Jest
import { render, screen, fireEvent, waitFor } from '@testing-library/react';
import userEvent from '@testing-library/user-event';
import '@testing-library/jest-dom';

// Component testing pattern
describe('UserProfile', () => {
  it('should display user information', () => {
    const user = { id: 1, name: 'Alice', email: 'alice@example.com' };
    render(<UserProfile user={user} />);

    expect(screen.getByText('Alice')).toBeInTheDocument();
    expect(screen.getByText('alice@example.com')).toBeInTheDocument();
  });

  it('should handle user interactions', async () => {
    const
    render(<UserForm />);

    // User interactions
    await userEvent.type(screen.getByLabelText('Name'), 'Bob');
    await userEvent.click(screen.getByRole('button', { name: 'Submit' }));

    await waitFor(() => {
      expect(onSubmit).toHaveBeenCalledWith({ name: 'Bob' });
    });
  });
});

// Hook testing
import { renderHook, act } from '@testing-library/react';

test('useCounter hook', () => {
  const { result } = renderHook(() => useCounter(0));

  expect(result.current.count).toBe(0);

  act(() => {
    result.current.increment();
  });

  expect(result.current.count).toBe(1);
});

// Context and Provider testing
const wrapper = ({ children }: { children: React.ReactNode }) => (
  <AuthProvider>{children}</AuthProvider>
);

test('useAuth hook with context', () => {
  const { result } = renderHook(() => useAuth(), { wrapper });
  expect(result.current.user).toBeDefined();
});

Quick Vitest Comparison (Inlined for Standalone Use)

When to Choose Vitest over Jest:

• New Vite/Vite-based projects (Next.js with Turbopack, SvelteKit)
• Need faster test execution (10-100x faster)
• ESM-first architecture
• Hot Module Replacement for tests

When to Stick with Jest:

• Existing large codebases with Jest already configured
• Corporate environments with established Jest workflows
• Need mature ecosystem and extensive plugins
• React apps with Create React App (default Jest setup)

Migration Snippet (Jest → Vitest):

// Jest: import from '@testing-library/jest-dom'
import '@testing-library/jest-dom';

// Vitest: import from vitest globals
import { expect, test, describe } from 'vitest';
import { screen } from '@testing-library/react';

// Most Jest syntax works in Vitest unchanged
test('component renders', () => {
  render(<Component />);
  expect(screen.getByText('Hello')).toBeTruthy();
});

[Full TypeScript, React, and Vitest patterns available in respective skills if deployed together]

Summary

• Jest is the industry standard with 70% market share
• TypeScript support via ts-jest with full type safety
• All-in-one solution: Test runner, assertions, mocks, coverage
• React Testing Library integration for component testing
• Mature ecosystem with extensive tooling and support
• Snapshot testing for UI regression testing
• Migration path from Vitest with compatible API
• Perfect for: Existing projects, corporate environments, React apps, legacy support
• Trade-off: Slower than Vitest but more mature and widely supported