Parsing request body (JSON and form data) in Node.js - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output of this Express.js JSON body parser example?

Consider this Express.js route that uses the built-in JSON parser middleware. What will be the value of req.body after a POST request with JSON {"name":"Alice","age":30}?

Node.js

import express from 'express';
const app = express();
app.use(express.json());
app.post('/user', (req, res) => {
  res.json(req.body);
});

A {"name":"Alice","age":30}

B "{\"name\":\"Alice\",\"age\":30}"

C {}

D undefined

💡 Hint

Think about what express.json() middleware does to the incoming request body.

The express.json() middleware parses the incoming JSON string and converts it into a JavaScript object. So req.body will be the parsed object {name: 'Alice', age: 30}.

📝 Syntax

intermediate

Which option correctly parses URL-encoded form data in Express.js?

You want to parse form data sent with content type application/x-www-form-urlencoded. Which middleware usage is correct?

Node.js

import express from 'express';
const app = express();
// Middleware to parse form data here
app.post('/submit', (req, res) => {
  res.json(req.body);
});

A app.use(express.form());

B app.use(express.json({ extended: true }));

C app.use(express.urlencoded());

D app.use(express.urlencoded({ extended: true }));

💡 Hint

Check the correct middleware function name and options for parsing URL-encoded data.

The express.urlencoded() middleware parses URL-encoded form data. The extended: true option allows rich objects and arrays to be encoded.

🔧 Debug

advanced

Why does req.body remain empty when sending JSON data?

Given this Express.js setup, why is req.body empty after sending JSON data in a POST request?

Node.js

import express from 'express';
const app = express();
app.post('/data', (req, res) => {
  res.json(req.body);
});
app.use(express.json());

A express.json() does not parse JSON data by default; you must enable it explicitly.

B The request must have content type text/plain for express.json() to work.

C The middleware express.json() is used after the route, so it doesn't parse the body before the route handler.

D req.body is always empty unless you use express.urlencoded() middleware.

💡 Hint

Middleware order matters in Express.js.

Middleware in Express runs in the order it is declared. Since express.json() is declared after the route, the route handler runs before the body is parsed, so req.body is empty.

❓ state_output

advanced

What is the value of req.body after sending multipart/form-data without middleware?

You send a POST request with Content-Type: multipart/form-data but do not use any middleware to parse it. What will req.body contain?

Node.js

import express from 'express';
const app = express();
app.post('/upload', (req, res) => {
  res.json(req.body);
});

A Parsed form fields as an object

B undefined

C Raw multipart data as a string

D {} (empty object)

💡 Hint

Express does not parse multipart/form-data by default.

Express does not parse multipart/form-data automatically. Without middleware like multer, req.body is undefined.

🧠 Conceptual

expert

Which middleware setup correctly handles JSON and URL-encoded form data in Express.js?

You want your Express.js app to accept both JSON and URL-encoded form data in POST requests. Which setup correctly parses both types before route handlers?

A app.use(express.json()); app.use(express.urlencoded({ extended: false }));

B app.use(express.urlencoded({ extended: true })); app.use(express.json());

C app.use(express.json({ extended: true })); app.use(express.urlencoded());

D app.use(express.urlencoded()); app.use(express.json({ limit: '1mb' }));

💡 Hint

Check the correct order and options for both middlewares.

The correct setup is to use express.json() and express.urlencoded() with proper options. The order does not strictly matter, but both must be included. The extended option is required for express.urlencoded(). Option A uses valid syntax and options.