POST route handling in Express - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output when sending JSON data to this POST route?

Consider this Express POST route that receives JSON data and responds with a message including the user's name.

What will the server respond with if the client sends {"name": "Alice"} in the request body?

Express

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

app.post('/greet', (req, res) => {
  const userName = req.body.name;
  res.send(`Hello, ${userName}!`);
});

A "Hello, Alice!"

B "Hello, undefined!"

C SyntaxError: Unexpected token in JSON

D Error: Cannot read property 'name' of undefined

💡 Hint

Remember to use express.json() middleware to parse JSON bodies.

The express.json() middleware parses the JSON body and makes it available as req.body. Since the client sends {"name": "Alice"}, req.body.name is 'Alice', so the response is 'Hello, Alice!'.

📝 Syntax

intermediate

Which option correctly defines a POST route that reads form data?

You want to create a POST route in Express that reads URL-encoded form data sent by a client.

Which code snippet correctly sets up the middleware and route handler?

A

app.use(express.urlencoded());
app.post('/submit', (req, res) => {
  res.send(req.body.username);
});

B

app.use(express.json());
app.post('/submit', (req, res) => {
  res.send(req.body.username);
});

C

app.post('/submit', (req, res) => {
  res.send(req.body.username);
});

D

app.use(express.urlencoded({ extended: true }));
app.post('/submit', (req, res) => {
  res.send(req.body.username);
});

💡 Hint

URL-encoded form data needs a specific middleware with extended option.

To parse URL-encoded form data, express.urlencoded middleware with { extended: true } is required. Option D correctly uses this middleware and accesses req.body.username.

🔧 Debug

advanced

Why does this POST route handler cause an error when receiving JSON?

Examine this Express POST route code:

Why does sending JSON data to '/data' cause the server to crash with 'TypeError: Cannot read property 'id' of undefined'?

Express

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

app.post('/data', (req, res) => {
  const id = req.body.id;
  res.send(`ID is ${id}`);
});

A Because express.json() middleware is missing, so req.body is undefined.

B Because req.body.id is not a valid property name.

C Because the route path '/data' is incorrect for POST requests.

D Because res.send() cannot send template literals.

💡 Hint

Check if the server can parse JSON bodies.

Without express.json() middleware, Express does not parse JSON request bodies, so req.body is undefined. Accessing req.body.id causes a TypeError.

❓ state_output

advanced

What is the final value of 'count' after these POST requests?

This Express app counts how many times the '/click' POST route is called.

After three POST requests to '/click', what is the value of 'count'?

Express

const express = require('express');
const app = express();
let count = 0;

app.post('/click', (req, res) => {
  count += 1;
  res.send(`Count is ${count}`);
});

A 0

B 3

C 1

D undefined

💡 Hint

The variable 'count' is outside the route handler and increments on each call.

The variable 'count' starts at 0 and increments by 1 on each POST request to '/click'. After 3 requests, count is 3.

🧠 Conceptual

expert

Which statement about Express POST route middleware order is true?

In Express, you want to handle POST requests with JSON bodies and also log each request.

Which statement correctly describes the middleware order to ensure both logging and JSON parsing work properly?

A Middleware order does not matter; Express runs all middleware in parallel.

B express.json() middleware must come before logging middleware to parse the body first.

C Logging middleware should be placed before express.json() middleware to log raw request data.

D Logging middleware should be placed after the route handler to log response data.

💡 Hint

Think about when you want to see the original request data in logs.

Middleware runs in the order defined. To log raw request data, logging middleware should come before express.json() which modifies req.body. This way logs show original data.