Multer middleware setup in Express - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

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

Consider this Multer setup in an Express app:

const multer = require('multer');
const upload = multer({
  limits: { fileSize: 1000 }
});

app.post('/upload', upload.single('file'), (req, res) => {
  res.send('File uploaded');
});

What will happen if a user uploads a file of 2000 bytes?

Express

const multer = require('multer');
const upload = multer({
  limits: { fileSize: 1000 }
});

app.post('/upload', upload.single('file'), (req, res) => {
  res.send('File uploaded');
});

A The server crashes due to unhandled file size.

B The server responds with 'File uploaded' and saves the file.

C The file is partially saved up to 1000 bytes and then truncated.

D Multer throws a 'LIMIT_FILE_SIZE' error and the request is rejected.

💡 Hint

Think about how Multer handles file size limits and errors.

Multer enforces the file size limit and throws a 'LIMIT_FILE_SIZE' error if exceeded. The upload is rejected and the route handler is not called.

📝 Syntax

intermediate

Which Multer setup correctly stores files in 'uploads/' folder?

Choose the correct Multer configuration to save uploaded files to the 'uploads/' directory with original filenames.

A const upload = multer({ storage: multer.diskStorage({ destination: 'uploads/', filename: (req, file, cb) => cb(null, file.originalname) }) });

B const upload = multer({ storage: multer.memoryStorage() });

C const upload = multer({ storage: multer.diskStorage({ destination: (req, file, cb) => cb(null, 'uploads/'), filename: (req, file, cb) => cb(null, file.originalname) }) });

D const upload = multer({ dest: 'uploads/' });

💡 Hint

Remember that destination can be a function or a string, but filename must be a function.

Option A correctly uses diskStorage with destination as a string (valid) and filename callback to save files with original names in 'uploads/'. Option A saves files with random names. Option A also works but uses a function for destination. Option A stores files in memory, not disk.

🔧 Debug

advanced

Why does this Multer setup not save files to disk?

Review this Multer setup:

const multer = require('multer');
const storage = multer.diskStorage({
  destination: 'uploads/',
  filename: (req, file, cb) => {
    cb(null, file.originalname);
  }
});
const upload = multer({ storage });

Files are not saved to disk after upload. What is the cause?

Express

const multer = require('multer');
const storage = multer.diskStorage({
  destination: 'uploads/',
  filename: (req, file, cb) => {
    cb(null, file.originalname);
  }
});
const upload = multer({ storage });

A The 'storage' object is not passed correctly to multer.

B The 'filename' callback is missing the 'file' parameter.

C The 'uploads/' folder does not exist, causing failure.

D The 'destination' option must be a function, not a string.

💡 Hint

Multer requires the destination directory to exist beforehand.

Multer does not create the 'uploads/' folder automatically. If the directory does not exist, the file save operation fails and no file is written to disk.

❓ state_output

advanced

What is the value of req.file after upload?

Given this route:

const upload = multer({ dest: 'uploads/' });

app.post('/upload', upload.single('avatar'), (req, res) => {
  res.json(req.file);
});

A user uploads a file named 'photo.png'. What does req.file contain?

Express

const upload = multer({ dest: 'uploads/' });

app.post('/upload', upload.single('avatar'), (req, res) => {
  res.json(req.file);
});

A A string with the file path 'uploads/photo.png'.

B An object with fields like originalname: 'photo.png', filename: random string, path: 'uploads/filename', size, mimetype, etc.

C Undefined, because multer does not add req.file.

D An array of files uploaded under 'avatar'.

💡 Hint

Check what multer.single adds to the request object.

Multer adds a 'file' object with details about the uploaded file, including originalname, filename (random), path, size, and mimetype.

🧠 Conceptual

expert

Which Multer option prevents uploading files with wrong MIME types?

You want to accept only image files (jpeg, png) in your upload route. Which Multer option helps reject other file types before saving?

A Use the 'fileFilter' option with a function that checks file.mimetype and calls cb(null, false) for invalid types.

B Set 'limits.fileSize' to restrict file size to images only.

C Use 'storage' option with diskStorage and check file extension in 'filename' callback.

D Use 'dest' option to specify folder and rely on client-side validation.

💡 Hint

Think about how to reject files based on type before saving.

The 'fileFilter' option lets you check the file's mimetype and accept or reject it by calling the callback accordingly. This prevents unwanted files from being saved.