Testing REST APIs: Unit Tests and Integration Tests

Testing REST APIs: Unit Tests and Integration Tests

Untested REST APIs are technical debt waiting to explode. Every endpoint change risks breaking existing clients. A comprehensive test suite catches regressions before deployment, documents expected behavior, and gives you confidence to refactor.

Setting Up Jest and Supertest

npm install --save-dev jest supertest

Add to package.json:

"scripts": {
  "test": "jest --coverage",
  "test:watch": "jest --watchAll"
},
"jest": {
  "testEnvironment": "node",
  "coverageThreshold": {
    "global": { "lines": 80 }
  }
}

Integration Testing with Supertest

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

describe('GET /api/users', () => {
  it('should return 200 and a list of users', async () => {
    const res = await request(app)
      .get('/api/users')
      .set('Authorization', `Bearer ${testToken}`);

    expect(res.statusCode).toBe(200);
    expect(res.body.success).toBe(true);
    expect(Array.isArray(res.body.data)).toBe(true);
  });

  it('should return 401 without authentication', async () => {
    const res = await request(app).get('/api/users');
    expect(res.statusCode).toBe(401);
  });
});

What to Test

Test the happy path (successful operations), all error conditions (400, 401, 403, 404, 409, 422), edge cases (empty arrays, null values, boundary values), and authorization (that users cannot access resources they should not). Every endpoint should have at least three tests: success, not-found, and unauthorized.