File size limits in Express - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What happens when a file larger than the limit is uploaded?

Consider an Express app using the express.json() middleware with a file size limit set to 1MB. What will happen if a client uploads a JSON payload of 2MB?

Express

const express = require('express');
const app = express();
app.use(express.json({ limit: '1mb' }));
app.post('/upload', (req, res) => {
  res.send('Upload successful');
});

A The server accepts the upload and processes the full 2MB payload without error.

B The server truncates the payload to 1MB and processes only that part.

C The server rejects the request and responds with a 413 Payload Too Large error.

D The server crashes due to memory overflow.

💡 Hint

Think about how Express middleware handles payloads exceeding the set limit.

Express's built-in body parsers like express.json() enforce the size limit and respond with a 413 error if the payload is too large.

📝 Syntax

intermediate

Which option correctly sets a 5MB file size limit for JSON payloads?

Choose the correct way to configure Express's JSON parser to accept payloads up to 5 megabytes.

A app.use(express.json({ limit: 5 }));

B app.use(express.json({ maxSize: 5 * 1024 * 1024 }));

C app.use(express.json({ sizeLimit: '5MB' }));

D app.use(express.json({ limit: '5mb' }));

💡 Hint

Check the official option name and format for size limits in Express.

The correct option is 'limit' with a string value like '5mb'. Other option names or formats are invalid.

🔧 Debug

advanced

Why does this Express app not enforce the file size limit?

Given the code below, why does the server accept payloads larger than 1MB without error?

Express

const express = require('express');
const app = express();
app.use(express.urlencoded({ extended: true }));
app.use(express.json());
app.post('/data', (req, res) => {
  res.send('Received');
});

A The server needs a separate middleware to enforce size limits.

B The file size limit is missing from express.json(), so the default unlimited size is used.

C The limit option must be set on both express.urlencoded() and express.json() to work.

D express.urlencoded() overrides express.json() and disables size limits.

💡 Hint

Look at the middleware configuration for express.json().

Without specifying the 'limit' option, express.json() uses the default limit which is quite large, effectively allowing big payloads.

🧠 Conceptual

advanced

How does Express handle file uploads exceeding the limit with multer?

When using multer middleware to handle file uploads with a size limit set, what happens if a user uploads a file larger than the limit?

A Multer throws a MulterError with code 'LIMIT_FILE_SIZE' and does not save the file.

B Multer ignores the limit and saves the full file.

C Multer automatically truncates the file to the limit and saves it.

D Multer crashes the server with an unhandled exception.

💡 Hint

Think about how multer signals errors for file size violations.

Multer emits a MulterError with code 'LIMIT_FILE_SIZE' when the uploaded file exceeds the configured size limit and prevents saving the file.

❓ state_output

expert

What is the value of req.file after a too-large file upload with multer?

In an Express route using multer with a 1MB file size limit, a user uploads a 2MB file. What will be the value of req.file inside the route handler?

Express

const multer = require('multer');
const upload = multer({ limits: { fileSize: 1 * 1024 * 1024 } });
app.post('/upload', upload.single('file'), (req, res) => {
  res.json({ file: req.file });
});

A undefined, because multer rejects the file and does not set req.file.

B An object containing file details for the uploaded file.

C null, indicating no file was uploaded.

D An empty object {}.

💡 Hint

Consider multer's behavior when a file exceeds the size limit.

When multer rejects a file due to size, it does not set req.file, so req.file remains undefined.