Unit Testing with Jest

npm install --save-dev jest
# or for TypeScript:
npm install --save-dev jest @types/jest ts-jest

// src/utils/math.js
export function add(a, b) { return a + b; }
export function divide(a, b) {
  if (b === 0) throw new Error('Division by zero');
  return a / b;
}

// src/utils/math.test.js
import { add, divide } from './math';

describe('math utils', () => {
  test('adds two numbers', () => {
    expect(add(2, 3)).toBe(5);
  });

  test('divides two numbers', () => {
    expect(divide(10, 2)).toBe(5);
    expect(divide(10, 2)).toBeCloseTo(5);
  });

  test('throws on divide by zero', () => {
    expect(() => divide(10, 0)).toThrow('Division by zero');
  });
});

// Mocking modules
import * as api from './api';

jest.mock('./api');

test('calls the API once', async () => {
  api.fetchUser.mockResolvedValue({ id: 1, name: 'Bo' });

  const user = await api.fetchUser(1);

  expect(api.fetchUser).toHaveBeenCalledTimes(1);
  expect(api.fetchUser).toHaveBeenCalledWith(1);
  expect(user.name).toBe('Bo');
});