Jest Testing Cheat Sheet

Basic Test Structure

// math.test.js
describe('Math operations', () => {
  test('adds 1 + 2 to equal 3', () => {
    expect(1 + 2).toBe(3);
  });

  it('subtracts 5 - 2 to equal 3', () => {
    expect(5 - 2).toBe(3);
  });
});

// Setup and teardown
describe('Database tests', () => {
  beforeAll(() => {
    // Runs once before all tests
    return initializeDatabase();
  });

  beforeEach(() => {
    // Runs before each test
    return clearDatabase();
  });

  afterEach(() => {
    // Runs after each test
  });

  afterAll(() => {
    // Runs once after all tests
    return closeDatabase();
  });
});

Matchers

Async Testing

// Promises
test('fetches user data', () => {
  return fetchUser().then(data => {
    expect(data.name).toBe('John');
  });
});

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

// Resolves/Rejects
test('promise resolves', () => {
  return expect(fetchUser()).resolves.toBe('data');
});

test('promise rejects', () => {
  return expect(fetchError()).rejects.toThrow('error');
});

Mocking

// Mock functions
const mockFn = jest.fn();
mockFn.mockReturnValue(42);
mockFn.mockResolvedValue('async value');

expect(mockFn).toHaveBeenCalled();
expect(mockFn).toHaveBeenCalledWith(arg1, arg2);
expect(mockFn).toHaveBeenCalledTimes(2);

// Mock modules
jest.mock('./api');
const { fetchData } = require('./api');
fetchData.mockResolvedValue({ data: 'mocked' });

// Spy on methods
const spy = jest.spyOn(object, 'method');
expect(spy).toHaveBeenCalled();
spy.mockRestore();

Snapshot Testing

import renderer from 'react-test-renderer';
import Component from './Component';

test('renders correctly', () => {
  const tree = renderer
    .create(<Component />)
    .toJSON();
  expect(tree).toMatchSnapshot();
});

// Update snapshots: jest -u