Supertest for HTTP assertions in Express - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output of this Supertest assertion?

Given the Express app below, what will the Supertest assertion return?

Express

const express = require('express');
const app = express();
app.get('/hello', (req, res) => {
  res.status(200).json({message: 'Hello World'});
});

const request = require('supertest');

request(app)
  .get('/hello')
  .expect('Content-Type', /json/)
  .expect(200)
  .then(response => console.log(response.body))
  .catch(err => console.error(err));

A {"message":"Hello World"}

B SyntaxError

C {"message":"hello world"}

D TypeError

💡 Hint

Check the JSON response body and status code returned by the endpoint.

The Express route returns a JSON object with message 'Hello World' and status 200. Supertest checks for JSON content type and status 200, so the response body logged is {message: 'Hello World'}.

📝 Syntax

intermediate

Which option causes a syntax error in Supertest usage?

Identify which Supertest code snippet will cause a syntax error.

Express

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

A request(app).get('/').expect(200).end((err, res) => { if (err) throw err; });

B request(app).post('/login').send({user: 'test'}).expect(200).then(res => console.log(res.body));

C request(app).get('/').expect(200).then(res => console.log(res.body);

D request(app).get('/').expect('Content-Type', /json/).expect(200);

💡 Hint

Look carefully at parentheses and semicolons in the code.

Option C is missing a closing parenthesis for the then() method, causing a syntax error.

🔧 Debug

advanced

Why does this Supertest test fail with a timeout?

Consider this test code: const request = require('supertest'); const app = require('./app'); test('GET /data returns 200', () => { return request(app) .get('/data') .expect(200); }); Why might this test fail with a timeout error?

A The Express app does not call res.end() or res.send(), so the response never finishes.

B The test is missing a done callback parameter.

C Supertest does not support promises, so .expect(200) is invalid.

D The route '/data' does not exist, so it returns 404 immediately.

💡 Hint

Think about what happens if the server never sends a response.

If the Express route handler does not send or end the response, Supertest waits indefinitely, causing a timeout.

❓ state_output

advanced

What is the value of res.body after this Supertest request?

Given this Express route and Supertest request, what is the value of res.body?

Express

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

app.post('/sum', (req, res) => {
  const {a, b} = req.body;
  res.json({result: a + b});
});

const request = require('supertest');

request(app)
  .post('/sum')
  .send({a: 5, b: 7})
  .expect(200)
  .then(res => console.log(res.body));

A {}

B {"result":"57"}

C {"a":5,"b":7}

D {"result":12}

💡 Hint

Check how the route calculates the sum and what it sends back.

The route extracts a and b from the JSON body, adds them as numbers, and returns {result: 12}.

🧠 Conceptual

expert

Which option best explains why Supertest can test Express apps without a running server?

Why can Supertest send HTTP requests to an Express app instance without calling app.listen()?

A Supertest requires the app to be running on localhost before tests can run.

B Supertest uses Node.js http module to inject requests directly into the Express app's request handler function.

C Supertest automatically starts a hidden server on a random port before sending requests.

D Supertest mocks the Express app methods to simulate responses without real HTTP traffic.

💡 Hint

Think about how Supertest integrates with Express internally.

Supertest creates HTTP requests that call the Express app's handler function directly, bypassing the need for a real network server.