Testing Express with Supertest

Supertest drives an Express app through real HTTP semantics without binding a port — it wraps `app` and asserts on status, headers, and body. This tests the full middleware stack, not isolated handler functions.

					import request from 'supertest';
import { app } from '../app';

it('creates a user', async () => {
  const res = await request(app)
    .post('/api/users')
    .set('Authorization', 'Bearer test-token')
    .send({ email: 'a@b.com', age: 25 })
    .expect(201);
  expect(res.body.id).toBeDefined();
});
				

Export the Express app without `listen()` for testability. Mock external dependencies (DB, queues) at module boundaries or inject fakes via factory. Test failure paths: 400 validation, 401 auth, 404 not found, 500 mapped errors.

On interviews: contrast Supertest integration tests with unit tests on services, how to seed fixtures, and avoiding flaky tests from shared mutable state.

Common pitfalls: calling `listen()` in the app module, sharing one DB without transactions or cleanup, and asserting only happy paths.

The trade-off is balancing simplicity, performance, safety, and operability — name which axis you optimized and what cost you accepted.

Checklist:

• Export app separately from server bootstrap.
• Mock or isolate external IO.
• Cover auth, validation, and error responses.
• Reset state between tests reliably.