Testing with Jest and Supertest

Why Test?

• Catch bugs before production
• Refactor with confidence
• Document how code works
• Prevent regressions

Setup

npm install --save-dev jest supertest

{
  "scripts": {
    "test": "jest"
  },
  "jest": {
    "testEnvironment": "node"
  }
}

Unit Tests

// utils.js
function add(a, b) {
  return a + b;
}

function isValidEmail(email) {
  return /^[^\s@]+@[^\s@]+\.[^\s@]+$/.test(email);
}

module.exports = { add, isValidEmail };

// utils.test.js
const { add, isValidEmail } = require('./utils');

describe('Math utilities', () => {
  test('adds two numbers', () => {
    expect(add(2, 3)).toBe(5);
    expect(add(-1, 1)).toBe(0);
  });
});

describe('Email validation', () => {
  test('validates correct email', () => {
    expect(isValidEmail('test@example.com')).toBe(true);
  });
  
  test('rejects invalid email', () => {
    expect(isValidEmail('invalid')).toBe(false);
  });
});

API Testing

// app.js
const express = require('express');
const app = express();

app.use(express.json());

app.get('/api/users', (req, res) => {
  res.json([{ id: 1, name: 'John' }]);
});

app.post('/api/users', (req, res) => {
  res.status(201).json({ id: 2, ...req.body });
});

module.exports = app;

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

describe('GET /api/users', () => {
  test('returns list of users', async () => {
    const response = await request(app)
      .get('/api/users')
      .expect(200)
      .expect('Content-Type', /json/);
    
    expect(response.body).toHaveLength(1);
    expect(response.body[0]).toHaveProperty('name', 'John');
  });
});

describe('POST /api/users', () => {
  test('creates new user', async () => {
    const newUser = { name: 'Jane', email: 'jane@example.com' };
    
    const response = await request(app)
      .post('/api/users')
      .send(newUser)
      .expect(201);
    
    expect(response.body).toMatchObject(newUser);
    expect(response.body).toHaveProperty('id');
  });
});

Mocking

const User = require('./models/User');

jest.mock('./models/User');

describe('getUser', () => {
  test('returns user by id', async () => {
    const mockUser = { id: 1, name: 'John' };
    User.findById.mockResolvedValue(mockUser);
    
    const user = await getUser(1);
    
    expect(User.findById).toHaveBeenCalledWith(1);
    expect(user).toEqual(mockUser);
  });
});

Key Takeaway

Write unit tests for functions, API tests for endpoints. Mock external dependencies. Use beforeEach for test isolation. Aim for 80% code coverage. Run tests in CI/CD.