Jest

Test Structure and Naming Conventions

describe('Calculator', () => {
  test('adds two numbers', () => {
    expect(1 + 2).toBe(3);
  });

  // `it` is an alias for `test`
  it('subtracts two numbers', () => {
    expect(5 - 2).toBe(3);
  });

  beforeEach(() => {
    // runs before every test in this describe block
  });

  afterEach(() => {
    // runs after every test in this describe block
  });
});

• Test files are named *.test.js or live under a __tests__/ folder.
• describe groups related tests; test/it defines an individual case.

Common Assertions and Matchers

expect(2 + 2).toBe(4); // strict equality (===)
expect({ name: 'Ada' }).toEqual({ name: 'Ada' }); // deep equality
expect([1, 2, 3]).toContain(2);
expect('hello world').toMatch(/world/);
expect(null).toBeNull();
expect(undefined).toBeUndefined();
expect(true).toBeTruthy();
expect(0).toBeFalsy();
expect(() => {
  throw new Error('fail');
}).toThrow('fail');

// Async assertions
await expect(fetchUser(1)).resolves.toEqual({ id: 1 });
await expect(fetchUser(-1)).rejects.toThrow();

Mocking Basics

const mockFn = jest.fn();
mockFn('a');
expect(mockFn).toHaveBeenCalledWith('a');
expect(mockFn).toHaveBeenCalledTimes(1);

// Mock a return value
const getUser = jest.fn().mockReturnValue({ id: 1, name: 'Ada' });

// Mock an entire module
jest.mock('./api');
import { fetchUser } from './api';
fetchUser.mockResolvedValue({ id: 1, name: 'Ada' });

// Spy on an existing method, keeping its real implementation optional
const spy = jest.spyOn(console, 'log').mockImplementation(() => {});

Running and Filtering Tests from the CLI

npx jest                      # run every test file
npx jest user.test.js           # run a single file
npx jest -t "adds two numbers"    # run tests matching a name pattern
npx jest --watch                    # re-run on file changes
npx jest --coverage                   # generate a coverage report

References

• Jest Documentation
• Expect API Reference
• Mock Functions