Testing GET endpoints in Express - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the response body of this GET endpoint?

Consider this Express GET endpoint. What will the client receive as the response body when requesting /hello?

Express

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

app.get('/hello', (req, res) => {
  res.json({ message: 'Hello, world!' });
});

A {"message":"Hello, world!"}

B "Hello, world!"

C {"msg":"Hello, world!"}

D Status 404 Not Found

💡 Hint

Look at the method used to send the response and the object passed.

The endpoint uses res.json() to send a JSON object with a key 'message' and value 'Hello, world!'. So the client receives {"message":"Hello, world!"}.

📝 Syntax

intermediate

Which option correctly defines a GET endpoint that sends plain text?

Which code snippet correctly defines a GET endpoint at /text that sends the plain text response "Hello"?

A app.get('/text', (req, res) => { res.json('Hello'); });

B app.get('/text', (req, res) => { res.sendJson('Hello'); });

C app.get('/text', (req, res) => { res.send('Hello'); });

D app.get('/text', (req, res) => { res.sendText('Hello'); });

💡 Hint

Check the Express method that sends plain text responses.

res.send() sends plain text or HTML. res.json() sends JSON. res.sendJson and res.sendText are not valid Express methods.

🔧 Debug

advanced

Why does this GET endpoint cause a runtime error?

Examine the code below. Why will a GET request to /data cause a runtime error?

Express

app.get('/data', (req, res) => {
  const data = undefined;
  res.json(data.value);
});

A TypeError because data is undefined and accessing data.value fails

B ReferenceError because res is not defined

C SyntaxError due to missing semicolon

D No error, response is null

💡 Hint

What happens if you try to access a property of undefined?

Accessing a property on undefined causes a TypeError at runtime.

❓ state_output

advanced

What is the response after multiple GET requests?

Given this Express app, what will be the response body of the third GET request to /count?

Express

let count = 0;
app.get('/count', (req, res) => {
  count += 1;
  res.json({ count });
});

A {"count":2}

B {"count":1}

C {"count":0}

D {"count":3}

💡 Hint

The variable count increases by 1 on each request.

Each GET request increments count by 1. On the third request, count is 3.

🧠 Conceptual

expert

Which option correctly explains why this GET endpoint never sends a response?

Consider this Express GET endpoint. Why does a client request to /never hang indefinitely?

Express

app.get('/never', (req, res) => {
  if (false) {
    res.send('Done');
  }
  // no else or res.send outside if
});

A Because the if condition is false, res.send throws an error

B Because res.send is never called, the server never ends the response, causing the client to wait forever

C Because Express requires res.json instead of res.send

D Because the route path is invalid and Express ignores the handler

💡 Hint

Think about what happens if no response method is called in Express.

If no response method like res.send or res.end is called, the client waits forever because the server never finishes the response.