Multiple file uploads in Express - Mini Project: Build & Apply

Multiple file uploads

📖 Scenario: You are building a simple Express server that allows users to upload multiple files at once. This is useful for applications like photo galleries or document management where users need to send several files in one go.

🎯 Goal: Create an Express server that accepts multiple file uploads using the multer middleware. You will set up the data structure, configure multer for multiple files, write the route to handle uploads, and finalize the server to listen on a port.

📋 What You'll Learn

Create an Express app instance

Configure multer to accept multiple files with the field name 'photos'

Write a POST route '/upload' that handles multiple file uploads

Start the Express server on port 3000

💡 Why This Matters

🌍 Real World

Many web applications need to let users upload multiple files at once, such as photo albums, resumes, or project documents.

💼 Career

Understanding how to handle multiple file uploads is a common requirement for backend developers working with Node.js and Express.

1

DATA SETUP: Create Express app and import multer

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

Express

// Your code here to import express and multer, and create app

Hint

Use require('express') and require('multer') to import the modules, then call express() to create the app.

2

CONFIGURATION: Set up multer storage and upload handler

Create a multer storage configuration using multer.diskStorage() that saves files to a folder named uploads. Then create an upload variable using multer({ storage }).

Express

const express = require('express');
const multer = require('multer');
const app = express();

// Your code here to configure multer storage and upload

Hint

Use multer.diskStorage() to set destination and filename. Then pass this storage to multer().

3

CORE LOGIC: Create POST route to handle multiple file uploads

Add a POST route at /upload that uses upload.array('photos', 5) middleware to accept up to 5 files with the field name photos. Inside the route handler, send a JSON response with { message: 'Files uploaded successfully' }.

Express

const express = require('express');
const multer = require('multer');
const app = express();

const storage = multer.diskStorage({
  destination: function (req, file, cb) {
    cb(null, 'uploads/');
  },
  filename: function (req, file, cb) {
    cb(null, Date.now() + '-' + file.originalname);
  }
});

const upload = multer({ storage });

// Your code here to add POST /upload route with upload.array('photos', 5)

Hint

Use app.post with upload.array('photos', 5) middleware and send a JSON response inside the handler.

4

COMPLETION: Start the Express server on port 3000

Add code to make the Express app listen on port 3000 and log 'Server started on port 3000' when running.

Express

const express = require('express');
const multer = require('multer');
const app = express();

const storage = multer.diskStorage({
  destination: function (req, file, cb) {
    cb(null, 'uploads/');
  },
  filename: function (req, file, cb) {
    cb(null, Date.now() + '-' + file.originalname);
  }
});

const upload = multer({ storage });

app.post('/upload', upload.array('photos', 5), (req, res) => {
  res.json({ message: 'Files uploaded successfully' });
});

// Your code here to start server on port 3000

Hint

Use app.listen(3000, () => { console.log(...) }) to start the server.

Practice

1. In Express, which method from multer is used to handle multiple file uploads at once?

easy

A. upload.array()

B. upload.single()

C. upload.fields()

D. upload.none()

Solution

1. Step 1: Understand multer upload methods

   Multer provides different methods: single() for one file, array() for multiple files, and fields() for multiple fields.

2. Step 2: Identify method for multiple files

   To upload multiple files under the same field name, upload.array() is used.

3. Final Answer:

   upload.array() -> Option A

4. Quick Check:

   Multiple files = upload.array() [OK]

Hint: Multiple files use upload.array() method in multer [OK]

Common Mistakes:

• Using upload.single() for multiple files
• Confusing upload.fields() with upload.array()
• Not specifying max count in upload.array()

2. Which of the following is the correct syntax to accept up to 5 files named 'photos' using multer in Express?

easy

A. upload.array('photos', 5)

B. upload.single('photos', 5)

C. upload.fields([{ name: 'photos', maxCount: 5 }])

D. upload.none('photos', 5)

Solution

1. Step 1: Recall multer syntax for multiple files

   The method upload.array(fieldname, maxCount) accepts multiple files with the same field name and limits the count.

2. Step 2: Match syntax to question

   To accept up to 5 files named 'photos', upload.array('photos', 5) is correct.

3. Final Answer:

   upload.array('photos', 5) -> Option A

4. Quick Check:

   Max 5 files = upload.array('photos', 5) [OK]

Hint: Use upload.array('fieldname', maxCount) for multiple files [OK]

Common Mistakes:

• Using upload.single() with max count
• Confusing upload.fields() syntax
• Using upload.none() incorrectly

3. Given this Express route using multer:

app.post('/upload', upload.array('docs', 3), (req, res) => {
  res.send(req.files.length);
});

What will be the response if a user uploads 2 files named 'docs'?

medium

A. 3

B. Error

C. 1

D. 2

Solution

1. Step 1: Understand upload.array behavior

   The middleware upload.array('docs', 3) allows up to 3 files. If user uploads 2 files, both are accepted.

2. Step 2: Check response code

   The handler sends req.files.length, which is the number of uploaded files, here 2.

3. Final Answer:

   2 -> Option D

4. Quick Check:

   Uploaded files count = 2 [OK]

Hint: req.files.length shows how many files were uploaded [OK]

Common Mistakes:

• Assuming maxCount is always returned
• Confusing req.file with req.files
• Expecting error if fewer than maxCount files uploaded

4. What is wrong with this Express route for multiple file uploads?

app.post('/upload', upload.array('files'), (req, res) => {
  console.log(req.file);
  res.send('Uploaded');
});

medium

A. upload.array() cannot be used for multiple files

B. upload.array() requires a maxCount parameter

C. Should use req.files instead of req.file to access multiple files

D. res.send() must send JSON, not string

Solution

1. Step 1: Check multer usage for multiple files

   When using upload.array(), uploaded files are stored in req.files (an array), not req.file.

2. Step 2: Identify error in code

   The code logs req.file, which will be undefined or incorrect for multiple files.

3. Final Answer:

   Should use req.files instead of req.file to access multiple files -> Option C

4. Quick Check:

   Multiple files = req.files [OK]

Hint: Use req.files (plural) for multiple files, not req.file [OK]

Common Mistakes:

• Forgetting req.files vs req.file difference
• Assuming maxCount is mandatory
• Thinking upload.array() is invalid

5. You want to accept multiple files from two different fields: 'images' (max 3 files) and 'documents' (max 2 files). Which multer setup correctly handles this?

hard

A. upload.array('images', 3), upload.array('documents', 2)

B. upload.fields([{ name: 'images', maxCount: 3 }, { name: 'documents', maxCount: 2 }])

C. upload.single('images'), upload.single('documents')

D. upload.none()

Solution

1. Step 1: Understand multer methods for multiple fields

   To handle multiple fields with multiple files each, use upload.fields() with an array of objects specifying field names and max counts.

2. Step 2: Analyze options

   upload.fields([{ name: 'images', maxCount: 3 }, { name: 'documents', maxCount: 2 }]) correctly uses upload.fields([{ name: 'images', maxCount: 3 }, { name: 'documents', maxCount: 2 }]). upload.array('images', 3), upload.array('documents', 2) is invalid syntax as you cannot chain two upload.array() calls in one route.

3. Final Answer:

   upload.fields([{ name: 'images', maxCount: 3 }, { name: 'documents', maxCount: 2 }]) -> Option B

4. Quick Check:

   Multiple fields = upload.fields() [OK]

Hint: Use upload.fields() for multiple fields with multiple files [OK]

Common Mistakes:

• Trying to chain multiple upload.array() calls
• Using upload.single() for multiple files
• Using upload.none() which disables file uploads