Test-driven development supports translating this sentence into English.

Test-Driven Development (TDD) is a software development methodology that emphasizes writing test cases before writing the actual code. This approach allows developers to define requirements more clearly, reduce errors, and improve code maintainability. Express, as a popular Node.js framework, combined with TDD, can significantly enhance the efficiency and quality of backend service development.

The Basic Process of Test-Driven Development

The core workflow of TDD follows the "Red-Green-Refactor" cycle:

1. Red: Write a failing test case that describes the functional requirement.
2. Green: Write the minimal amount of code to make the test pass.
3. Refactor: Optimize the code structure while keeping the tests passing.

For example, implementing a simple API endpoint in Express:

// Test case (using Jest)
test('GET /api/hello should return "world"', async () => {
  const res = await request(app).get('/api/hello');
  expect(res.statusCode).toBe(200);
  expect(res.body.message).toBe('world');
});

At this point, running the test will fail (red) because the route has not yet been implemented.

Practical TDD in Express

Initializing the Test Environment

Install necessary dependencies:

npm install jest supertest --save-dev

Configure Jest test scripts:

// package.json
"scripts": {
  "test": "jest --watchAll"
}

Example of Route Testing

Implementing a TDD workflow for a user registration route:

1. Write the test first:

describe('POST /api/users', () => {
  it('should create a new user', async () => {
    const mockUser = { name: 'Alice', email: 'alice@example.com' };
    const res = await request(app)
      .post('/api/users')
      .send(mockUser);
    expect(res.status).toBe(201);
    expect(res.body).toHaveProperty('id');
  });
});

2. Write the minimal implementation:

// routes/users.js
router.post('/', (req, res) => {
  const { name, email } = req.body;
  res.status(201).json({ id: Date.now(), name, email });
});

Middleware Testing

Testing a typical authentication middleware scenario:

// Test case
test('should block unauthenticated requests', async () => {
  const res = await request(app).get('/api/protected');
  expect(res.status).toBe(401);
});

// Middleware implementation
function authMiddleware(req, res, next) {
  if (!req.headers.authorization) {
    return res.sendStatus(401);
  }
  next();
}

Advanced TDD Techniques

Database Integration Testing

Using an in-memory database (e.g., SQLite) for isolated testing:

beforeEach(async () => {
  await sequelize.sync({ force: true });
});

test('user creation should persist to DB', async () => {
  await User.create({ name: 'Bob' });
  const users = await User.findAll();
  expect(users.length).toBe(1);
});

Error Handling Testing

Validating exception handling logic:

test('should return 400 for invalid input', async () => {
  const res = await request(app)
    .post('/api/users')
    .send({ invalidField: 'value' });
  expect(res.status).toBe(400);
  expect(res.body.error).toBeDefined();
});

Common Issues and Solutions

Test Speed Optimization

• Use jest.setTimeout to adjust timeout for asynchronous tests.
• Disable parallel testing with --runInBand to resolve resource conflicts.

Test Coverage

Configure Jest to collect coverage:

// package.json
"jest": {
  "collectCoverage": true,
  "coveragePathIgnorePatterns": ["/node_modules/", "/tests/"]
}

Mocking External Services

Use Jest's mock functionality to simulate third-party APIs:

jest.mock('axios');
axios.get.mockResolvedValue({ data: { success: true } });

test('should handle external API response', async () => {
  const res = await request(app).get('/api/external');
  expect(res.body.success).toBe(true);
});

TDD in Continuous Integration

Integrate tests into CI/CD pipelines:

# GitHub Actions example
name: Node CI
on: [push]
jobs:
  test:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v2
      - run: npm install
      - run: npm test

The Testing Pyramid in Express Applications

A rational test layering strategy:

1. Unit Tests: Target individual middleware/utility functions.
2. Integration Tests: Validate interactions between routes and databases.
3. E2E Tests: Simulate full API request chains.

Example unit test:

// utils/formatDate.test.js
test('should format ISO string to human-readable', () => {
  expect(formatDate('2023-01-01')).toBe('January 1, 2023');
});