Request body transformation in Express - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to parse JSON request bodies in Express.

Express

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

app.use(express.[1]());

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

Drag options to blanks, or click blank then click option'

Aurlencoded

Bjson

Ctext

Draw

Attempts:

3 left

💡 Hint

Common Mistakes

Using express.urlencoded() instead of express.json() for JSON data.

Forgetting to add the middleware before route handlers.

Express uses express.json() middleware to parse JSON request bodies.

2fill in blank

medium

Complete the code to transform the request body by adding a new property before sending the response.

Express

app.post('/transform', (req, res) => {
  const data = req.body;
  data.[1] = 'processed';
  res.json(data);
});

Drag options to blanks, or click blank then click option'

Aresult

Btype

Cprocessed

Dstatus

Attempts:

3 left

💡 Hint

Common Mistakes

Using a value as a property name instead of a key.

Trying to modify req.body directly without assigning to a variable.

We add a new property named status to the request body object.

3fill in blank

hard

Fix the error in the code to correctly parse URL-encoded form data.

Express

app.use(express.[1]({ extended: true }));

Drag options to blanks, or click blank then click option'

Ajson

Btext

Curlencoded

Draw

Attempts:

3 left

💡 Hint

Common Mistakes

Using express.json() for form data.

Omitting the middleware and causing req.body to be undefined.

To parse URL-encoded form data, use express.urlencoded() middleware.

4fill in blank

hard

Fill both blanks to create a middleware that transforms the request body by uppercasing a 'name' property.

Express

app.use((req, res, next) => {
  if (req.body && req.body.[1]) {
    req.body.[2] = req.body.name.toUpperCase();
  }
  next();
});

Drag options to blanks, or click blank then click option'

Aname

Busername

Cuser

DName

Attempts:

3 left

💡 Hint

Common Mistakes

Using the wrong property name in the condition.

Not calling next() to continue the middleware chain.

The middleware checks if req.body.name exists, then creates a new property username with the uppercase value.

5fill in blank

hard

Fill all three blanks to create a route that transforms the request body by adding a timestamp and sending the modified object.

Express

app.post('/submit', (req, res) => {
  const data = req.body;
  data.[1] = new Date().[2]();
  res.[3](data);
});

Drag options to blanks, or click blank then click option'

Atimestamp

BtoISOString

Cjson

Dnow

Attempts:

3 left

💡 Hint

Common Mistakes

Using Date.now() which returns a number instead of a string.

Forgetting to send the response with res.json().

The code adds a timestamp property with the current date in ISO string format, then sends the data as JSON.

Practice

1. What is the main purpose of request body transformation in Express?

easy

A. To send the response back to the client

B. To handle errors in the server

C. To change incoming data into a more useful format before using it

D. To set HTTP headers for the response

Solution

1. Step 1: Understand request body transformation

   Request body transformation modifies the data received from the client to a better format for the app.

2. Step 2: Identify the main purpose

   This process happens after parsing and helps clean or convert data before use.

3. Final Answer:

   To change incoming data into a more useful format before using it -> Option C

4. Quick Check:

   Request body transformation = change data format [OK]

Hint: Think: transform means change data format before use [OK]

Common Mistakes:

• Confusing transformation with sending response
• Mixing transformation with error handling
• Thinking it sets HTTP headers

2. Which Express middleware is commonly used to parse JSON request bodies before transformation?

easy

A. express.urlencoded()

B. express.static()

C. express.raw()

D. express.json()

Solution

1. Step 1: Identify JSON parsing middleware

   express.json() parses incoming JSON request bodies into JavaScript objects.

2. Step 2: Confirm other options

   express.urlencoded() parses URL-encoded data, express.static() serves files, express.raw() parses raw buffer data.

3. Final Answer:

   express.json() -> Option D

4. Quick Check:

   JSON body parser = express.json() [OK]

Hint: Use express.json() to parse JSON bodies before transforming [OK]

Common Mistakes:

• Using express.urlencoded() for JSON data
• Confusing express.static() as parser
• Not parsing body before transformation

3. Given this Express code, what will be the value of req.body.age after transformation?

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

If the client sends JSON {"age": "30"}?

medium

A. 30 (number)

B. "30" (string)

C. NaN

D. undefined

Solution

1. Step 1: Parse JSON body

   express.json() converts the JSON string {"age": "30"} into an object with age as string "30".

2. Step 2: Transform age to number

   req.body.age = Number(req.body.age) converts the string "30" to the number 30.

3. Final Answer:

   30 (number) -> Option A

4. Quick Check:

   Number("30") = 30 [OK]

Hint: Number() converts string digits to number type [OK]

Common Mistakes:

• Assuming age stays a string
• Thinking Number("30") returns NaN
• Confusing undefined with missing property

4. Identify the error in this Express request body transformation code:

app.use(express.json());
app.post('/data', (req, res) => {
  req.body.count = parseInt(req.body.count);
  if (!req.body.count) {
    res.status(400).send('Count required');
  }
  res.send(req.body);
});

medium

A. parseInt should be replaced with Number

B. The check '!req.body.count' fails when count is 0

C. express.json() middleware is missing

D. res.send() should be called before res.status()

Solution

1. Step 1: Understand parseInt and check

   parseInt converts strings to numbers; 0 is falsy in JavaScript.

2. Step 2: Analyze the if condition

   The condition '!req.body.count' treats 0 as false, wrongly triggering the error for count=0.

3. Final Answer:

   The check '!req.body.count' fails when count is 0 -> Option B

4. Quick Check:

   Falsy check fails for 0 count [OK]

Hint: Check for 0 explicitly, not with '!value' [OK]

Common Mistakes:

• Assuming parseInt is wrong here
• Missing express.json() middleware
• Calling res.send before res.status

5. You want to transform an incoming request body to remove any properties with empty strings and convert all numeric strings to numbers. Which code snippet correctly performs this transformation in Express?

hard

A. app.use(express.json());
app.post('/transform', (req, res) => {
for (const key in req.body) {
if (req.body[key] === '') delete req.body[key];
else if (!isNaN(req.body[key])) req.body[key] = Number(req.body[key]);
}
res.send(req.body);
});

B. app.use(express.json());
app.post('/transform', (req, res) => {
Object.keys(req.body).forEach(key => {
if (req.body[key] === '') req.body[key] = null;
else if (typeof req.body[key] === 'string') req.body[key] = parseInt(req.body[key]);
});
res.send(req.body);
});

C. app.use(express.urlencoded());
app.post('/transform', (req, res) => {
for (const key in req.body) {
if (req.body[key] === '') delete req.body[key];
else if (!isNaN(req.body[key])) req.body[key] = Number(req.body[key]);
}
res.send(req.body);
});

D. app.use(express.json());
app.post('/transform', (req, res) => {
for (const key in req.body) {
if (req.body[key] === '') req.body[key] = undefined;
else if (isNaN(req.body[key])) req.body[key] = Number(req.body[key]);
}
res.send(req.body);
});

Solution

1. Step 1: Check middleware and iteration

   express.json() is needed to parse JSON bodies; for..in loops over keys correctly.

2. Step 2: Validate transformation logic

   app.use(express.json()); app.post('/transform', (req, res) => { for (const key in req.body) { if (req.body[key] === '') delete req.body[key]; else if (!isNaN(req.body[key])) req.body[key] = Number(req.body[key]); } res.send(req.body); }); deletes keys with empty strings and converts numeric strings to numbers using !isNaN check.

3. Step 3: Compare other options

   app.use(express.json()); app.post('/transform', (req, res) => { Object.keys(req.body).forEach(key => { if (req.body[key] === '') req.body[key] = null; else if (typeof req.body[key] === 'string') req.body[key] = parseInt(req.body[key]); }); res.send(req.body); }); sets empty strings to null and always parses strings with parseInt (may convert non-numeric wrongly). app.use(express.urlencoded()); app.post('/transform', (req, res) => { for (const key in req.body) { if (req.body[key] === '') delete req.body[key]; else if (!isNaN(req.body[key])) req.body[key] = Number(req.body[key]); } res.send(req.body); }); uses express.urlencoded() which is wrong for JSON. app.use(express.json()); app.post('/transform', (req, res) => { for (const key in req.body) { if (req.body[key] === '') req.body[key] = undefined; else if (isNaN(req.body[key])) req.body[key] = Number(req.body[key]); } res.send(req.body); }); wrongly converts non-numeric strings to numbers.

4. Final Answer:

   app.use(express.json()); app.post('/transform', (req, res) => { for (const key in req.body) { if (req.body[key] === '') delete req.body[key]; else if (!isNaN(req.body[key])) req.body[key] = Number(req.body[key]); } res.send(req.body); }); -> Option A

5. Quick Check:

   Delete empty and convert numeric strings = app.use(express.json()); app.post('/transform', (req, res) => { for (const key in req.body) { if (req.body[key] === '') delete req.body[key]; else if (!isNaN(req.body[key])) req.body[key] = Number(req.body[key]); } res.send(req.body); }); [OK]

Hint: Delete empty strings, convert numeric strings with !isNaN check [OK]

Common Mistakes:

• Using wrong middleware for JSON
• Setting empty strings to null instead of deleting
• Converting non-numeric strings to numbers