Request body transformation in Express - Mini Project: Build & Apply

Request Body Transformation in Express

📖 Scenario: You are building a simple Express server that receives user data in JSON format. You want to transform the incoming request body to add a new field before processing it further.

🎯 Goal: Build an Express server that accepts JSON data in a POST request, transforms the request body by adding a new field receivedAt with the current timestamp, and then sends back the transformed data as JSON.

📋 What You'll Learn

Create an Express app with JSON body parsing middleware

Add a POST route at /submit to receive JSON data

Add a receivedAt field with the current ISO timestamp to the request body

Send the transformed request body back as JSON in the response

💡 Why This Matters

🌍 Real World

Transforming request bodies is common in APIs to add metadata or normalize data before processing.

💼 Career

Backend developers often need to manipulate incoming data in Express servers to prepare it for storage or further logic.

1

Setup Express app and JSON middleware

Create an Express app by requiring express and calling express(). Use app.use(express.json()) to enable JSON body parsing.

Express

const express = require('express');
const app = express();
// Your code here

Hint

Use express.json() middleware to parse JSON bodies automatically.

2

Add POST route at /submit

Add a POST route handler on app for the path '/submit' using app.post('/submit', (req, res) => { ... }).

Express

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

// Your code here

Hint

Use app.post to create a POST route at /submit.

3

Transform request body by adding receivedAt

Inside the POST route handler, add a new field receivedAt to req.body with the current ISO timestamp using new Date().toISOString().

Express

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

app.post('/submit', (req, res) => {
  // Your code here
});

Hint

Use new Date().toISOString() to get the current time in ISO format.

4

Send transformed body as JSON response

Send the transformed req.body back as JSON in the response using res.json(req.body).

Express

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

app.post('/submit', (req, res) => {
  req.body.receivedAt = new Date().toISOString();
  // Your code here
});

Hint

Use res.json() to send a JSON response.

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