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

File Size Limits in Express

📖 Scenario: You are building a simple Express server that accepts JSON data uploads. To keep your server safe and efficient, you want to limit the size of the incoming JSON files.

🎯 Goal: Create an Express server that limits the JSON request body size to 100kb. This will prevent users from sending very large JSON payloads.

📋 What You'll Learn

Create an Express app instance called app

Set up JSON body parsing with a size limit of 100kb using express.json()

Add a POST route at /upload that responds with status 200 and message 'Upload received' when JSON is accepted

Start the server listening on port 3000

💡 Why This Matters

🌍 Real World

Limiting JSON body size is important in real-world web servers to prevent abuse and crashes from very large requests.

💼 Career

Backend developers often configure body parsers and set limits to ensure server stability and security.

1

Create Express app

Create a variable called express by requiring the 'express' module. Then create an Express app instance called app by calling express().

Express

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

Hint

Use require('express') to import Express, then call it as a function to create the app.

2

Configure JSON body parser with size limit

Use app.use() to add JSON body parsing middleware with a size limit of '100kb' by calling express.json({ limit: '100kb' }).

Express

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

Hint

Use express.json() with the limit option inside app.use().

3

Add POST route to accept uploads

Add a POST route at /upload using app.post('/upload', (req, res) => { ... }). Inside the handler, respond with status 200 and send the text 'Upload received'.

Express

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

Hint

Use app.post with the path '/upload' and send a 200 status with the message inside the callback.

4

Start the server on port 3000

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

Express

const express = require('express');
const app = express();
app.use(express.json({ limit: '100kb' }));
app.post('/upload', (req, res) => {
  res.status(200).send('Upload received');
});
// 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 file size limit in an Express app using body parsers?

easy

A. To change the file type of uploads

B. To allow unlimited file uploads without restrictions

C. To automatically compress uploaded files

D. To prevent users from uploading files that are too large and slow down the server

Solution

1. Step 1: Understand file size limits in Express

   File size limits stop very large files from being uploaded, protecting server resources.

2. Step 2: Identify the main reason for limits

   Limits keep the app safe and fast by preventing overload from big files.

3. Final Answer:

   To prevent users from uploading files that are too large and slow down the server -> Option D

4. Quick Check:

   File size limits = prevent large uploads [OK]

Hint: File size limits protect server from big uploads [OK]

Common Mistakes:

• Thinking limits compress files automatically
• Believing limits allow unlimited uploads
• Confusing file type changes with size limits

2. Which of the following is the correct way to set a 1MB file size limit using Express's built-in JSON body parser?

easy

A. app.use(express.json({ sizeLimit: 1 }))

B. app.use(express.json({ limit: '1mb' }))

C. app.use(express.json({ maxFileSize: '1MB' }))

D. app.use(express.json({ limit: 1000 }))

Solution

1. Step 1: Recall Express JSON parser options

   The correct option to set size limit is 'limit' with a string like '1mb'.

2. Step 2: Check each option's syntax

   Only app.use(express.json({ limit: '1mb' })) uses 'limit' with correct string format '1mb'. Others use wrong keys or units.

3. Final Answer:

   app.use(express.json({ limit: '1mb' })) -> Option B

4. Quick Check:

   Use 'limit' with string size in Express [OK]

Hint: Use 'limit' option with string size like '1mb' [OK]

Common Mistakes:

• Using wrong option names like sizeLimit or maxFileSize
• Passing number without units as limit
• Using uppercase units incorrectly

3. Given this Express code snippet, what happens if a client uploads a JSON body larger than 500kb?

app.use(express.json({ limit: '500kb' }));
app.post('/upload', (req, res) => {
  res.send('Upload successful');
});

medium

A. The server throws an error and does not call the route handler

B. The server ignores the size limit and processes the full upload

C. The server truncates the JSON to 500kb and processes it

D. The server accepts the upload and responds with 'Upload successful'

Solution

1. Step 1: Understand Express JSON parser behavior with limits

   If the JSON body exceeds the limit, Express throws a 'PayloadTooLargeError' and skips the route handler.

2. Step 2: Analyze the code flow

   The route handler sends 'Upload successful' only if parsing succeeds, which won't happen if size is too big.

3. Final Answer:

   The server throws an error and does not call the route handler -> Option A

4. Quick Check:

   Exceeding limit causes error, no handler call [OK]

Hint: Uploads over limit cause error, no success response [OK]

Common Mistakes:

• Assuming server truncates large JSON
• Thinking handler runs despite error
• Believing limit is ignored

4. Identify the error in this Express setup that tries to limit JSON body size to 2MB:

app.use(express.json({ limit: 2 * 1024 * 1024 }));

medium

A. The limit value should be a string like '2mb', not a number

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

C. The multiplication is incorrect; it should be 2 * 1000 * 1000

D. The limit should be set in bytes as a Buffer, not a number

Solution

1. Step 1: Check express.json() limit option type

   The 'limit' option expects a string with units like '2mb', not a raw number.

2. Step 2: Analyze the given code

   Passing a number causes Express to ignore or misinterpret the limit, so it should be '2mb'.

3. Final Answer:

   The limit value should be a string like '2mb', not a number -> Option A

4. Quick Check:

   Limit must be string with units, not number [OK]

Hint: Use string with units for limit, not number [OK]

Common Mistakes:

• Passing number instead of string for limit
• Misunderstanding unit conversion
• Thinking limit option is unsupported

5. You want to limit file uploads to 3MB using the multer middleware in Express. Which code snippet correctly sets this limit and handles errors to inform users when files are too large?

hard

A. const upload = multer({ limits: { fileSize: 3 * 1024 * 1024 } });
app.post('/upload', upload.single('file'), (req, res) => {
res.send('File uploaded');
});

B. const upload = multer({ limit: '3mb' });
app.post('/upload', upload.single('file'), (req, res) => {
res.send('File uploaded');
});

C. const upload = multer({ limits: { fileSize: 3 * 1024 * 1024 } });
app.post('/upload', (req, res) => {
upload.single('file')(req, res, err => {
if (err) return res.status(400).send('File too large');
res.send('File uploaded');
});
});

D. const upload = multer({ fileSizeLimit: 3 * 1024 * 1024 });
app.post('/upload', upload.single('file'), (req, res) => {
res.send('File uploaded');
});

Solution

1. Step 1: Set file size limit correctly in multer

   The correct option is 'limits: { fileSize: size_in_bytes }'. const upload = multer({ limits: { fileSize: 3 * 1024 * 1024 } }); app.post('/upload', (req, res) => { upload.single('file')(req, res, err => { if (err) return res.status(400).send('File too large'); res.send('File uploaded'); }); }); and A use this correctly.

2. Step 2: Handle errors to inform users

   const upload = multer({ limits: { fileSize: 3 * 1024 * 1024 } }); app.post('/upload', (req, res) => { upload.single('file')(req, res, err => { if (err) return res.status(400).send('File too large'); res.send('File uploaded'); }); }); wraps upload.single in route handler with error callback to catch file size errors and respond properly.

3. Step 3: Compare options

   const upload = multer({ limits: { fileSize: 3 * 1024 * 1024 } }); app.post('/upload', upload.single('file'), (req, res) => { res.send('File uploaded'); }); lacks error handling, B uses wrong option 'limit', D uses invalid 'fileSizeLimit'.

4. Final Answer:

   Option C correctly sets limit and handles errors to inform users -> Option C

5. Quick Check:

   Multer limits + error callback = const upload = multer({ limits: { fileSize: 3 * 1024 * 1024 } }); app.post('/upload', (req, res) => { upload.single('file')(req, res, err => { if (err) return res.status(400).send('File too large'); res.send('File uploaded'); }); }); [OK]

Hint: Use limits.fileSize and error callback to handle large files [OK]

Common Mistakes:

• Using wrong option names like limit or fileSizeLimit
• Not handling errors to inform users
• Assuming multer auto-handles file size errors