Jest

Delightful JavaScript testing framework with zero configuration and rich mocking capabilities.

Quick Start

# Install
npm install -D jest @types/jest

# With TypeScript
npm install -D jest ts-jest @types/jest
npx ts-jest config:init

# Or with Babel
npm install -D jest @babel/preset-env @babel/preset-typescript

Configuration

jest.config.ts

import type { Config } from 'jest';

const config: Config = {
  preset: 'ts-jest',
  testEnvironment: 'node',
  roots: ['<rootDir>/src'],
  testMatch: ['**/__tests__/**/*.ts', '**/*.test.ts', '**/*.spec.ts'],
  moduleNameMapper: {
    '^@/(.*)$': '<rootDir>/src/$1',
  },
  setupFilesAfterEnv: ['<rootDir>/jest.setup.ts'],
  collectCoverageFrom: [
    'src/**/*.{ts,tsx}',
    '!src/**/*.d.ts',
    '!src/**/__tests__/**',
  ],
  coverageThreshold: {
    global: {
      branches: 80,
      functions: 80,
      lines: 80,
      statements: 80,
    },
  },
  clearMocks: true,
  verbose: true,
};

export default config;

For React (with Testing Library)

import type { Config } from 'jest';

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

export default config;

jest.setup.ts

import '@testing-library/jest-dom';

// Global mocks
global.fetch = jest.fn();

// Extend expect
expect.extend({
  toBeWithinRange(received: number, floor: number, ceiling: number) {
    const pass = received >= floor && received <= ceiling;
    return {
      pass,
      message: () =>
        `expected ${received} ${pass ? 'not ' : ''}to be within range ${floor} - ${ceiling}`,
    };
  },
});

declare global {
  namespace jest {
    interface Matchers<R> {
      toBeWithinRange(floor: number, ceiling: number): R;
    }
  }
}

Basic Tests

Test Structure

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

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

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

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

  describe('add', () => {
    it('should add two positive numbers', () => {
      expect(calculator.add(1, 2)).toBe(3);
    });

    it('should handle negative numbers', () => {
      expect(calculator.add(-1, -2)).toBe(-3);
    });

    test.each([
      [1, 2, 3],
      [0, 0, 0],
      [-1, 1, 0],
      [100, 200, 300],
    ])('add(%i, %i) should return %i', (a, b, expected) => {
      expect(calculator.add(a, b)).toBe(expected);
    });
  });

  describe('divide', () => {
    it('should throw when dividing by zero', () => {
      expect(() => calculator.divide(10, 0)).toThrow('Division by zero');
    });
  });
});

Common Matchers

// Equality
expect(value).toBe(expected);           // ===
expect(value).toEqual(expected);        // Deep equality
expect(value).toStrictEqual(expected);  // Deep + undefined props

// Truthiness
expect(value).toBeNull();
expect(value).toBeUndefined();
expect(value).toBeDefined();
expect(value).toBeTruthy();
expect(value).toBeFalsy();

// Numbers
expect(value).toBeGreaterThan(3);
expect(value).toBeGreaterThanOrEqual(3);
expect(value).toBeLessThan(5);
expect(value).toBeCloseTo(0.3, 5);  // Floating point

// Strings
expect(string).toMatch(/pattern/);
expect(string).toContain('substring');
expect(string).toHaveLength(5);

// Arrays
expect(array).toContain(item);
expect(array).toContainEqual(object);
expect(array).toHaveLength(3);

// Objects
expect(object).toHaveProperty('key');
expect(object).toHaveProperty('nested.key', value);
expect(object).toMatchObject({ key: value });

// Exceptions
expect(() => fn()).toThrow();
expect(() => fn()).toThrow('message');
expect(() => fn()).toThrow(ErrorType);

// Negation
expect(value).not.toBe(other);

// Asymmetric matchers
expect(value).toEqual(expect.any(Number));
expect(value).toEqual(expect.stringContaining('sub'));
expect(value).toEqual(expect.arrayContaining([1, 2]));
expect(value).toEqual(expect.objectContaining({ key: value }));

Async Testing

Promises

// Return promise
test('fetches user data', () => {
  return fetchUser(1).then((user) => {
    expect(user.name).toBe('Alice');
  });
});

// Async/await
test('fetches user data', async () => {
  const user = await fetchUser(1);
  expect(user.name).toBe('Alice');
});

// Resolves/rejects
test('resolves to user', async () => {
  await expect(fetchUser(1)).resolves.toEqual({ name: 'Alice' });
});

test('rejects with error', async () => {
  await expect(fetchUser(-1)).rejects.toThrow('User not found');
});

Callbacks

test('callback with done', (done) => {
  fetchDataWithCallback((error, data) => {
    try {
      expect(error).toBeNull();
      expect(data).toEqual({ success: true });
      done();
    } catch (e) {
      done(e);
    }
  });
});

Timers

jest.useFakeTimers();

test('delays execution', () => {
  const callback = jest.fn();

  delayedCall(callback, 1000);
  expect(callback).not.toHaveBeenCalled();

  jest.advanceTimersByTime(1000);
  expect(callback).toHaveBeenCalledTimes(1);
});

test('with modern timers', () => {
  jest.useFakeTimers({ advanceTimers: true });

  const callback = jest.fn();
  setTimeout(callback, 1000);

  jest.runAllTimers();
  expect(callback).toHaveBeenCalled();
});

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

Mocking

Function Mocks

// Create mock function
const mockFn = jest.fn();

// With implementation
const mockAdd = jest.fn((a, b) => a + b);

// Mock return values
mockFn.mockReturnValue(42);
mockFn.mockReturnValueOnce(1).mockReturnValueOnce(2);
mockFn.mockResolvedValue({ data: 'value' });
mockFn.mockRejectedValue(new Error('Failed'));

// Assertions
expect(mockFn).toHaveBeenCalled();
expect(mockFn).toHaveBeenCalledTimes(2);
expect(mockFn).toHaveBeenCalledWith('arg1', 'arg2');
expect(mockFn).toHaveBeenLastCalledWith('lastArg');
expect(mockFn).toHaveBeenNthCalledWith(1, 'firstCallArg');
expect(mockFn).toHaveReturnedWith(42);

Module Mocks

// __mocks__/axios.ts
const axios = {
  get: jest.fn(),
  post: jest.fn(),
  create: jest.fn(() => axios),
};
export default axios;

// In test file
jest.mock('axios');
import axios from 'axios';

const mockedAxios = axios as jest.Mocked<typeof axios>;

test('fetches data', async () => {
  mockedAxios.get.mockResolvedValue({ data: { users: [] } });

  const result = await fetchUsers();

  expect(mockedAxios.get).toHaveBeenCalledWith('/api/users');
  expect(result).toEqual({ users: [] });
});

Partial Mocks

// Mock specific exports
jest.mock('./utils', () => ({
  ...jest.requireActual('./utils'),
  expensiveOperation: jest.fn(),
}));

// Mock with factory
jest.mock('./database', () => {
  return {
    connect: jest.fn().mockResolvedValue(true),
    query: jest.fn(),
  };
});

Spy on Methods

const video = {
  play() {
    return true;
  },
};

test('plays video', () => {
  const spy = jest.spyOn(video, 'play');
  video.play();

  expect(spy).toHaveBeenCalled();
  spy.mockRestore();
});

// Spy on prototype
jest.spyOn(Date.prototype, 'toISOString').mockReturnValue('2024-01-01');

Manual Mocks

// __mocks__/fs.ts
const fs = jest.createMockFromModule('fs') as typeof import('fs');

let mockFiles: Record<string, string> = {};

fs.readFileSync = jest.fn((path: string) => {
  if (path in mockFiles) {
    return mockFiles[path];
  }
  throw new Error(`ENOENT: ${path}`);
});

(fs as any).__setMockFiles = (files: Record<string, string>) => {
  mockFiles = files;
};

export default fs;

Snapshot Testing

// Basic snapshot
test('renders correctly', () => {
  const tree = renderer.create(<Button label="Click me" />).toJSON();
  expect(tree).toMatchSnapshot();
});

// Inline snapshot
test('formats date', () => {
  expect(formatDate(new Date('2024-01-15'))).toMatchInlineSnapshot(
    `"January 15, 2024"`
  );
});

// Property matchers for dynamic values
test('creates user', () => {
  const user = createUser('Alice');
  expect(user).toMatchSnapshot({
    id: expect.any(String),
    createdAt: expect.any(Date),
  });
});

// Update snapshots: jest --updateSnapshot

Code Coverage

# Run with coverage
jest --coverage

# Coverage report options
jest --coverage --coverageReporters="text" --coverageReporters="html"

// jest.config.ts coverage options
{
  collectCoverage: true,
  coverageDirectory: 'coverage',
  coverageReporters: ['text', 'lcov', 'html'],
  collectCoverageFrom: [
    'src/**/*.{ts,tsx}',
    '!src/**/*.d.ts',
    '!src/index.ts',
  ],
  coverageThreshold: {
    global: {
      branches: 80,
      functions: 80,
      lines: 80,
      statements: 80,
    },
    './src/utils/': {
      branches: 100,
      statements: 100,
    },
  },
}

Testing React Components

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

test('submits form with user data', async () => {
  const handleSubmit = jest.fn();
  const user = userEvent.setup();

  render(<LoginForm onSubmit={handleSubmit} />);

  await user.type(screen.getByLabelText(/email/i), 'test@example.com');
  await user.type(screen.getByLabelText(/password/i), 'password123');
  await user.click(screen.getByRole('button', { name: /submit/i }));

  expect(handleSubmit).toHaveBeenCalledWith({
    email: 'test@example.com',
    password: 'password123',
  });
});

test('shows error on invalid input', async () => {
  render(<LoginForm />);

  fireEvent.click(screen.getByRole('button', { name: /submit/i }));

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

test('fetches and displays users', async () => {
  const mockUsers = [{ id: 1, name: 'Alice' }];
  global.fetch = jest.fn().mockResolvedValue({
    ok: true,
    json: () => Promise.resolve(mockUsers),
  });

  render(<UserList />);

  expect(screen.getByText(/loading/i)).toBeInTheDocument();

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

Running Tests

# Run all tests
jest

# Watch mode
jest --watch

# Run specific file
jest path/to/test.spec.ts

# Run tests matching pattern
jest --testNamePattern="should add"

# Run only changed files
jest --onlyChanged

# Parallel execution
jest --maxWorkers=4

# Debug mode
node --inspect-brk node_modules/.bin/jest --runInBand

Reference Files

• mocking-patterns.md - Advanced mocking techniques
• performance.md - Optimizing test performance