Integration vs unit test decision in Express - Practice Questions

Challenge - 5 Problems

🧠 Conceptual

intermediate

When to use unit tests in Express apps?

Which scenario best fits writing a unit test in an Express application?

A Testing the deployment process of the Express app on a cloud server.

B Testing the entire API endpoint including database calls and middleware together.

C Testing the integration between two microservices communicating over HTTP.

D Testing a single middleware function's logic in isolation without involving the server or database.

💡 Hint

Unit tests focus on small parts of code working alone.

Unit tests check small pieces like functions or middleware alone, without external dependencies like databases or servers.

🧠 Conceptual

intermediate

When to prefer integration tests in Express apps?

Which situation is best suited for an integration test in an Express app?

A Checking if a route handler correctly processes a request and saves data to the database.

B Verifying a utility function that formats dates returns the correct string.

C Testing if a single middleware function throws an error on invalid input.

D Checking if the Express app starts without syntax errors.

💡 Hint

Integration tests check multiple parts working together.

Integration tests verify that components like route handlers and databases work together correctly.

❓ component_behavior

advanced

What happens when you run this Express route unit test?

Consider this unit test for an Express middleware that adds a header. What will the test output be?

Express

const req = {};
const res = { setHeader: jest.fn() };
const next = jest.fn();

function addCustomHeader(req, res, next) {
  res.setHeader('X-Custom', '123');
  next();
}

addCustomHeader(req, res, next);

console.log(res.setHeader.mock.calls.length);

A 2

B 0

C Throws ReferenceError because jest is not defined

D 1

💡 Hint

Think about the environment where this code runs and what jest.fn() means.

jest.fn() is a Jest testing function. Without Jest environment, it causes ReferenceError.

❓ state_output

advanced

What is the final state after this integration test runs?

Given this Express route and test, what will be the value of userCount after the test?

Express

let userCount = 0;

const express = require('express');
const app = express();
app.use(express.json());

app.post('/users', (req, res) => {
  userCount += 1;
  res.status(201).send({ id: userCount });
});

// Simulate test
const request = require('supertest');

(async () => {
  await request(app).post('/users').send({ name: 'Alice' });
  await request(app).post('/users').send({ name: 'Bob' });
  console.log(userCount);
})();

A 0

B 2

C 1

D Throws error because supertest is not installed

💡 Hint

Each POST request increments userCount.

Two POST requests run, each increments userCount by 1, so final value is 2.

🔧 Debug

expert

Why does this integration test fail with timeout?

Look at this Express integration test code. Why does the test timeout instead of passing?

Express

const express = require('express');
const app = express();

app.get('/ping', (req, res) => {
  setTimeout(() => res.send('pong'), 1000);
});

const request = require('supertest');

test('GET /ping returns pong', async () => {
  const response = await request(app).get('/ping');
  expect(response.text).toBe('pong');
});

A The test times out because Jest timeout is less than 1000ms and the response is delayed.

B The test times out because the route handler never calls res.send.

C The test times out because supertest does not support async/await syntax.

D The test times out because the Express app is missing app.listen call.

💡 Hint

Consider how long the route delays response and Jest's default timeout.

The route delays response by 1 second. Jest default timeout is 5 seconds, so normally it should pass. But if Jest timeout is set lower, test fails. The most common cause is a timeout shorter than 1000ms.