Multiple file uploads in Express - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output when uploading multiple files with this Express route?

Consider this Express route using multer middleware to handle multiple file uploads:

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

app.post('/upload', upload.array('photos', 3), (req, res) => {
  res.json({ count: req.files.length });
});

If a client uploads 4 files named 'photos', what will be the JSON response?

Express

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

app.post('/upload', upload.array('photos', 3), (req, res) => {
  res.json({ count: req.files.length });
});

A {"count":3}

B {"count":4}

C Error: Too many files uploaded

D {"count":0}

💡 Hint

Look at the second argument to upload.array. It limits the number of files; exceeding it causes a LIMIT_FILE_COUNT error.

The upload.array('photos', 3) middleware limits uploads to 3 files. Uploading 4 files exceeds the limit, triggering a MulterError 'LIMIT_FILE_COUNT'. The route handler is not reached, resulting in an error instead of a JSON response.

📝 Syntax

intermediate

Which option correctly sets up multer to accept multiple files with different field names?

You want to accept multiple files uploaded under two different fields: 'avatar' (1 file) and 'gallery' (up to 5 files). Which code snippet correctly configures multer middleware for this?

A upload.array('avatar', 1, 'gallery', 5)

B upload.fields([{ name: 'avatar', maxCount: 1 }, { name: 'gallery', maxCount: 5 }])

C upload.single('avatar', 1).array('gallery', 5)

D upload.multiple(['avatar', 'gallery'], [1, 5])

💡 Hint

Check multer's documentation for handling multiple fields with different file counts.

The upload.fields method accepts an array of objects specifying each field name and maxCount. Other options are invalid multer syntax.

🔧 Debug

advanced

Why does this Express route fail to save multiple uploaded files?

Examine this Express route snippet:

app.post('/upload', upload.single('photos'), (req, res) => {
  console.log(req.files);
  res.send('Upload complete');
});

When uploading multiple files under the field 'photos', req.files is undefined. Why?

Express

app.post('/upload', upload.single('photos'), (req, res) => {
  console.log(req.files);
  res.send('Upload complete');
});

A Because the field name 'photos' is incorrect and must be 'photo'

B Because req.files is only set when no files are uploaded

C Because multer requires upload.array for single file uploads

D Because upload.single only processes one file and sets req.file, not req.files

💡 Hint

Check multer's difference between single and array methods.

upload.single handles one file and sets req.file. For multiple files, upload.array must be used, which sets req.files. Hence, req.files is undefined here.

❓ state_output

advanced

What is the value of req.files after this upload?

Given this Express route:

app.post('/upload', upload.fields([{ name: 'doc', maxCount: 2 }, { name: 'img', maxCount: 3 }]), (req, res) => {
  res.json(req.files);
});

If a client uploads 1 file under 'doc' and 2 files under 'img', what is the structure of req.files?

Express

app.post('/upload', upload.fields([{ name: 'doc', maxCount: 2 }, { name: 'img', maxCount: 3 }]), (req, res) => {
  res.json(req.files);
});

A {"doc": [FileObject], "img": [FileObject, FileObject]}

B [FileObject, FileObject, FileObject]

C {"doc": FileObject, "img": FileObject}

D []

💡 Hint

Recall how multer structures req.files when using fields method.

When using upload.fields, req.files is an object with keys for each field name. Each key maps to an array of file objects uploaded under that field.

🧠 Conceptual

expert

Which statement about handling multiple file uploads in Express with multer is TRUE?

Choose the correct statement about multer's handling of multiple file uploads in Express.

A upload.array(fieldname, maxCount) stores files in req.files as an array of file objects

B upload.single(fieldname) stores multiple files in req.files as an array

C upload.fields([{name, maxCount}]) stores files in req.file as a single object

D Multer automatically merges files from different fields into a single array in req.files

💡 Hint

Review multer's documentation on how it stores uploaded files in the request object.

upload.array stores multiple files under the same field name in req.files as an array. upload.single stores one file in req.file. upload.fields stores files in req.files as an object keyed by field names. Multer does not merge files from different fields into one array.