Mohin Sheikh

Posted on Dec 25

Writing Unit Tests for an Express.js Application: A Beginner-Friendly Guide

📖 Table of Contents

Introduction to Unit Testing
Why Unit Testing is Important
Setting Up the Testing Environment
Installing Testing Dependencies
Project Structure for Testing
Writing Unit Tests for Controllers
Writing Unit Tests for Middleware
Writing Unit Tests for Services
Running Unit Tests
Best Practices for Unit Testing
Conclusion

1. Introduction to Unit Testing

Unit testing involves testing individual units or components of an application in isolation to ensure they work as expected. In Express.js, unit tests typically focus on testing controllers, middlewares, and services.

2. Why Unit Testing is Important

Detects bugs early in development.
Ensures code behaves as expected.
Makes refactoring easier.
Improves code maintainability.
Provides documentation for how the code should work.

3. Setting Up the Testing Environment

Ensure your Express.js application is set up and functional.

If not, follow the Setup Guide.

4. Installing Testing Dependencies

Install the required testing libraries:

npm install --save-dev jest ts-jest @types/jest supertest @types/supertest

jest: Testing framework.
ts-jest: Jest configuration for TypeScript.
supertest: Library for testing HTTP APIs.

4.1 Configure Jest

Create a jest.config.ts file in your project root:

export default {
  preset: 'ts-jest',
  testEnvironment: 'node',
  moduleNameMapper: {
    '^@/(.*)$': '<rootDir>/src/$1',
  },
  moduleFileExtensions: ['ts', 'js', 'json'],
  testMatch: ['**/__tests__/**/*.test.ts'],
};

Add a test script to your package.json:

{
  "scripts": {
    "test": "jest --coverage"
  }
}

5. Project Structure for Testing

src/
├── controllers/
│   ├── userController.ts
├── services/
│   ├── userService.ts
├── middleware/
│   ├── authMiddleware.ts
└── __tests__/
    ├── controllers/
    │   ├── userController.test.ts
    ├── services/
    │   ├── userService.test.ts
    ├── middleware/
        ├── authMiddleware.test.ts

6. Writing Unit Tests for Controllers

6.1 Example Controller

File: src/controllers/userController.ts

import { Request, Response } from 'express';

export const getUser = (req: Request, res: Response) => {
  res.status(200).json({ id: 1, name: 'John Doe' });
};

6.2 Test Controller

File: src/__tests__/controllers/userController.test.ts

import { getUser } from '../../controllers/userController';
import { Request, Response } from 'express';

describe('User Controller - getUser', () => {
  it('should return user data with status 200', () => {
    const req = {} as Request;
    const res = {
      status: jest.fn().mockReturnThis(),
      json: jest.fn(),
    } as unknown as Response;

    getUser(req, res);

    expect(res.status).toHaveBeenCalledWith(200);
    expect(res.json).toHaveBeenCalledWith({ id: 1, name: 'John Doe' });
  });
});

Explanation:

Mocked Request and Response.
Called the getUser controller function.
Asserted status code and response data.

7. Writing Unit Tests for Middleware

7.1 Example Middleware

File: src/middleware/authMiddleware.ts

import { Request, Response, NextFunction } from 'express';

export const isAuthenticated = (req: Request, res: Response, next: NextFunction) => {
  if (req.headers.authorization === 'Bearer validToken') {
    next();
  } else {
    res.status(403).json({ message: 'Forbidden' });
  }
};

7.2 Test Middleware

File: src/__tests__/middleware/authMiddleware.test.ts

import { isAuthenticated } from '../../middleware/authMiddleware';
import { Request, Response, NextFunction } from 'express';

describe('Auth Middleware - isAuthenticated', () => {
  it('should call next if token is valid', () => {
    const req = { headers: { authorization: 'Bearer validToken' } } as Request;
    const res = {} as Response;
    const next = jest.fn();

    isAuthenticated(req, res, next);

    expect(next).toHaveBeenCalled();
  });

  it('should return 403 if token is invalid', () => {
    const req = { headers: { authorization: '' } } as Request;
    const res = {
      status: jest.fn().mockReturnThis(),
      json: jest.fn(),
    } as unknown as Response;
    const next = jest.fn();

    isAuthenticated(req, res, next);

    expect(res.status).toHaveBeenCalledWith(403);
    expect(res.json).toHaveBeenCalledWith({ message: 'Forbidden' });
  });
});

8. Writing Unit Tests for Services

8.1 Example Service

File: src/services/userService.ts

export const findUserById = (id: number) => {
  if (id === 1) {
    return { id: 1, name: 'John Doe' };
  }
  return null;
};

8.2 Test Service

File: src/__tests__/services/userService.test.ts

import { findUserById } from '../../services/userService';

describe('User Service - findUserById', () => {
  it('should return user data when valid ID is provided', () => {
    const user = findUserById(1);
    expect(user).toEqual({ id: 1, name: 'John Doe' });
  });

  it('should return null when invalid ID is provided', () => {
    const user = findUserById(2);
    expect(user).toBeNull();
  });
});

9. Running Unit Tests

Run your tests with:

npm run test

Sample Output:

 PASS  __tests__/controllers/userController.test.ts
 PASS  __tests__/middleware/authMiddleware.test.ts
 PASS  __tests__/services/userService.test.ts

Test Suites: 3 passed, 3 total
Tests:       6 passed, 6 total
Snapshots:   0 total
Time:        2.345 s

Check Test Coverage

npm run test -- --coverage

10. Best Practices for Unit Testing

Test one functionality per test case.
Mock external dependencies (e.g., database calls).
Ensure your tests are independent and isolated.
Use descriptive describe and it blocks.
Avoid testing implementation details.
Include edge cases in your tests.

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