How to write tests in Node.js?

npm install -D jest

// math.js
function add(a, b) { return a + b; }
function multiply(a, b) { return a * b; }
module.exports = { add, multiply };

// math.test.js
const { add, multiply } = require('./math');

describe('Math functions', () => {
  test('adds 1 + 2 to equal 3', () => {
    expect(add(1, 2)).toBe(3);
  });

  test('multiplies 2 * 3 to equal 6', () => {
    expect(multiply(2, 3)).toBe(6);
  });
});

const { fetchUser } = require('./api');

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

jest.mock('./database');
const db = require('./database');

test('creates user', async () => {
  db.save.mockResolvedValue({ id: 1, name: 'John' });

  const result = await createUser({ name: 'John' });

  expect(db.save).toHaveBeenCalledWith({ name: 'John' });
  expect(result.id).toBe(1);
});

npm install -D supertest

const request = require('supertest');
const app = require('./app');

describe('GET /api/users', () => {
  it('returns list of users', async () => {
    const res = await request(app)
      .get('/api/users')
      .expect(200);

    expect(res.body).toBeInstanceOf(Array);
  });
});