How to test an Express.js application?

Testing Express.js Applications

Testing Express apps involves unit tests for individual functions and integration tests for the full HTTP request/response cycle. The key tool is Supertest — it makes real HTTP requests to your app without starting a server.

Setup

bash

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

App Structure for Testing

Separate your Express app from the server start:

// app.js — export app, no listen()
const express = require('express');
const app = express();
app.use(express.json());
app.use('/users', usersRouter);
module.exports = app;

// server.js — only this file calls listen()
const app = require('./app');
app.listen(3000);

Integration Tests with Supertest

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

describe('Users API', () => {

  describe('GET /users', () => {
    it('should return 200 and array of users', async () => {
      const res = await request(app).get('/users');

      expect(res.status).toBe(200);
      expect(res.body).toHaveProperty('data');
      expect(Array.isArray(res.body.data)).toBe(true);
    });
  });

  describe('POST /users', () => {
    it('should create a user and return 201', async () => {
      const res = await request(app)
        .post('/users')
        .send({ name: 'Alice', email: 'alice@test.com' });

      expect(res.status).toBe(201);
      expect(res.body.data.name).toBe('Alice');
    });

    it('should return 400 when email is missing', async () => {
      const res = await request(app)
        .post('/users')
        .send({ name: 'Alice' }); // no email!

      expect(res.status).toBe(400);
      expect(res.body).toHaveProperty('error');
    });
  });

  describe('GET /users/:id', () => {
    it('should return 404 for non-existent user', async () => {
      const res = await request(app).get('/users/9999');
      expect(res.status).toBe(404);
    });
  });
});

Testing Protected Routes

const jwt = require('jsonwebtoken');

function generateTestToken(payload = { id: 1, role: 'user' }) {
  return jwt.sign(payload, process.env.JWT_SECRET, { expiresIn: '1h' });
}

describe('Protected Routes', () => {
  it('should return 401 without token', async () => {
    const res = await request(app).get('/profile');
    expect(res.status).toBe(401);
  });

  it('should return 200 with valid token', async () => {
    const token = generateTestToken();
    const res = await request(app)
      .get('/profile')
      .set('Authorization', `Bearer ${token}`);

    expect(res.status).toBe(200);
  });
});

Mocking Services

// Mock the database layer
jest.mock('../services/users.service', () => ({
  findAll: jest.fn().mockResolvedValue([
    { id: 1, name: 'Alice' },
    { id: 2, name: 'Bob' }
  ]),
  findById: jest.fn().mockResolvedValue({ id: 1, name: 'Alice' }),
  create: jest.fn().mockResolvedValue({ id: 3, name: 'Charlie' })
}));

const usersService = require('../services/users.service');

it('should call service.findAll once', async () => {
  await request(app).get('/users');
  expect(usersService.findAll).toHaveBeenCalledTimes(1);
});

Test Database Strategy

Approach	Speed	Isolation	Realism
Mock service layer	Fastest	High	Low
In-memory DB (SQLite)	Fast	Medium	Medium
Test database	Slower	Low (needs cleanup)	High
Docker container	Slowest	High	Highest

jest.config.js

module.exports = {
  testEnvironment: 'node',
  setupFilesAfterEach: ['./jest.setup.js'],
  coveragePathIgnorePatterns: ['/node_modules/'],
};

Summary

Use Supertest + Jest for testing Express apps. Keep your app in app.js (no listen()) so tests can import it directly. Test the full HTTP cycle: status codes, response body, headers, and authentication. Mock the database/service layer for fast, isolated unit tests.

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