Request size limits in Express - Mini Project: Build & Apply

Set Request Size Limits in Express

📖 Scenario: You are building a simple Express server that accepts JSON data from clients. To protect your server from very large requests that could slow it down or cause errors, you want to set a limit on the size of incoming JSON payloads.

🎯 Goal: Create an Express server that uses the built-in express.json() middleware with a request size limit of 100kb. This will ensure the server only accepts JSON requests up to 100 kilobytes in size.

📋 What You'll Learn

Create an Express app instance called app

Use express.json() middleware with the option limit: '100kb'

Add a POST route at /data that responds with status 200

Start the server listening on port 3000

💡 Why This Matters

🌍 Real World

Limiting request size is important to prevent clients from sending too much data, which can slow down or crash servers.

💼 Career

Backend developers often configure middleware like express.json() with size limits to build secure and reliable APIs.

1

Create Express app instance

Write code to import express and create an Express app instance called app.

Express

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

Hint

Use require('express') to import Express and then call express() to create the app.

2

Add JSON middleware with size limit

Add the JSON middleware to app using express.json() with the option { limit: '100kb' }.

Express

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

Hint

Use app.use() to add the JSON middleware with the limit option set to '100kb'.

3

Create POST route at /data

Create a POST route on app at path /data that sends a 200 status response with res.send('OK').

Express

const express = require('express');
const app = express();
app.use(express.json({ limit: '100kb' }));
// Your code here

Hint

Use app.post('/data', (req, res) => { ... }) to create the route and respond with res.send('OK').

4

Start server on port 3000

Add code to start the Express server listening on port 3000 using app.listen(3000).

Express

const express = require('express');
const app = express();
app.use(express.json({ limit: '100kb' }));
app.post('/data', (req, res) => {
  res.send('OK');
});
// Your code here

Hint

Use app.listen(3000) to start the server on port 3000.

Practice

1. What is the main purpose of setting a request size limit in Express middleware like express.json()?

easy

A. To increase the speed of the server by caching requests

B. To prevent very large requests from slowing down or crashing the server

C. To automatically compress large requests for faster processing

D. To allow unlimited request sizes for flexibility

Solution

1. Step 1: Understand the role of request size limits

   Request size limits help protect the server from very large requests that can slow it down or cause crashes.

2. Step 2: Identify the correct purpose in Express middleware

   The limit option in express.json() sets this size limit to keep the server safe and responsive.

3. Final Answer:

   To prevent very large requests from slowing down or crashing the server -> Option B

4. Quick Check:

   Request size limit = prevent server overload [OK]

Hint: Request size limits protect server from overload [OK]

Common Mistakes:

• Thinking it speeds up requests by caching
• Confusing size limit with compression
• Assuming unlimited size is better

2. Which of the following is the correct way to set a 10kb limit on JSON request bodies in Express?

easy

A. app.use(express.json({ sizeLimit: 10 }))

B. app.use(express.json(limit = 10))

C. app.use(express.json({ maxSize: '10kb' }))

D. app.use(express.json({ limit: '10kb' }))

Solution

1. Step 1: Recall the correct option name for size limit

   The correct option to set request size limit in express.json() is limit.

2. Step 2: Check the correct syntax for setting 10kb

   The value should be a string with units, like '10kb'. So { limit: '10kb' } is correct.

3. Final Answer:

   app.use(express.json({ limit: '10kb' })) -> Option D

4. Quick Check:

   Use limit: '10kb' option [OK]

Hint: Use limit option with string size like '10kb' [OK]

Common Mistakes:

• Using wrong option names like sizeLimit or maxSize
• Passing number without units
• Using assignment inside options object

3. Given this Express setup:

app.use(express.json({ limit: '5kb' }));
app.post('/data', (req, res) => {
  res.send('Received');
});

What happens if a client sends a JSON body of 10kb to /data?

medium

A. The server rejects the request with a 413 Payload Too Large error

B. The server ignores the body and responds with an empty object

C. The server crashes due to memory overflow

D. The server accepts the request and responds with 'Received'

Solution

1. Step 1: Understand the limit setting effect

   The limit is set to 5kb, so any request body larger than 5kb will be rejected.

2. Step 2: Identify Express behavior on large requests

   Express responds with a 413 Payload Too Large error when the body exceeds the limit.

3. Final Answer:

   The server rejects the request with a 413 Payload Too Large error -> Option A

4. Quick Check:

   Request > limit = 413 error [OK]

Hint: Requests over limit get 413 error response [OK]

Common Mistakes:

• Assuming server accepts large requests anyway
• Thinking server crashes instead of error response
• Believing server ignores body silently

4. Consider this Express code snippet:

app.use(express.json({ limit: 10000 }));

What is the problem with this code regarding request size limits?

medium

A. There is no problem; this code sets a 10kb limit correctly

B. The limit option is not supported by express.json()

C. The limit value is too small and will reject all requests

D. The limit value should be a string with units like '10kb', not a number

Solution

1. Step 1: Check the type of the limit option value

   The limit option accepts both numbers (in bytes) and strings with units like '10kb'.

2. Step 2: Understand the effect of passing a number

   Passing 10000 sets the limit to 10000 bytes (approximately 10kb), which works correctly.

3. Final Answer:

   There is no problem; this code sets a 10kb limit correctly -> Option A

4. Quick Check:

   limit: number = bytes [OK]

Hint: Limit accepts number (bytes) or string with units [OK]

Common Mistakes:

• Thinking it must be string with units only
• Thinking limit option is unsupported
• Assuming number means bytes automatically is wrong

5. You want to accept JSON requests up to 1mb but URL-encoded form data only up to 100kb in your Express app. Which setup correctly applies these limits?

hard

A. app.use(express.json({ limit: 100 * 1024 }));\napp.use(express.urlencoded({ limit: 1 * 1024 * 1024, extended: false }));

B. app.use(express.json({ maxSize: '1mb' }));\napp.use(express.urlencoded({ maxSize: '100kb', extended: true }));

C. app.use(express.json({ limit: '1mb' }));\napp.use(express.urlencoded({ limit: '100kb', extended: true }));

D. app.use(express.json({ limit: '100kb' }));\napp.use(express.urlencoded({ limit: '1mb', extended: true }));

Solution

1. Step 1: Set JSON request limit to 1mb correctly

   Use express.json({ limit: '1mb' }) to set JSON body limit to 1 megabyte.

2. Step 2: Set URL-encoded form data limit to 100kb correctly

   Use express.urlencoded({ limit: '100kb', extended: true }) to set form data limit to 100 kilobytes.

3. Step 3: Verify option names and values

   Both use the correct limit option with string sizes and proper extended flag for URL-encoded.

4. Final Answer:

   app.use(express.json({ limit: '1mb' })); app.use(express.urlencoded({ limit: '100kb', extended: true })); -> Option C

5. Quick Check:

   Use limit strings per middleware type [OK]

Hint: Set limits separately with correct units and options [OK]

Common Mistakes:

• Swapping limits between JSON and URL-encoded
• Using numbers instead of strings for limits
• Using wrong option names like maxSize