File type validation in Express - Mini Project: Build & Apply

File type validation

📖 Scenario: You are building a simple Express server that accepts file uploads. To keep the server safe and organized, you want to allow only image files of type PNG and JPEG.

🎯 Goal: Create an Express server that validates uploaded files to accept only PNG and JPEG images.

📋 What You'll Learn

Create an Express app with a POST route /upload to accept file uploads.

Use the multer middleware to handle file uploads.

Create a file filter function that only accepts files with MIME types image/png and image/jpeg.

Send a response indicating success or failure based on file type validation.

💡 Why This Matters

🌍 Real World

File type validation is important to prevent users from uploading harmful or unwanted files to your server. This keeps your app secure and organized.

💼 Career

Backend developers often need to handle file uploads safely. Knowing how to validate file types with Express and multer is a common task in web development jobs.

1

Set up Express app and multer

Create an Express app by requiring express and multer. Then create an instance of Express called app and set up multer with const upload = multer().

Express

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

// Your code here

Need a hint?

Use require('express') and require('multer') to import the modules. Then call express() to create the app and multer() to create the upload handler.

2

Create file filter function

Create a function called fileFilter that takes req, file, and cb as parameters. Inside, check if file.mimetype is either image/png or image/jpeg. If yes, call cb(null, true), else call cb(null, false).

Express

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

const app = express();

// Create fileFilter function here
const upload = multer();

Need a hint?

The fileFilter function controls which files multer accepts. Use file.mimetype to check the file type and call cb accordingly.

3

Add POST route with upload middleware

Add a POST route /upload to the Express app. Use upload.single('file') as middleware to handle a single file upload with the field name file. In the route handler, send a JSON response with { success: true } if req.file exists, else { success: false, message: 'Invalid file type' }.

Express

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

const app = express();

function fileFilter(req, file, cb) {
    if (file.mimetype === 'image/png' || file.mimetype === 'image/jpeg') {
        cb(null, true);
    } else {
        cb(null, false);
    }
}

const upload = multer({ fileFilter });

// Add POST /upload route here

Need a hint?

Use app.post to create the route. Use upload.single('file') to handle the file upload. Check req.file to see if the file passed the filter.

4

Start the Express server

Add code to start the Express server on port 3000 using app.listen. Log the message 'Server running on port 3000' when the server starts.

Express

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

const app = express();

function fileFilter(req, file, cb) {
    if (file.mimetype === 'image/png' || file.mimetype === 'image/jpeg') {
        cb(null, true);
    } else {
        cb(null, false);
    }
}

const upload = multer({ fileFilter });

app.post('/upload', upload.single('file'), (req, res) => {
    if (req.file) {
        res.json({ success: true });
    } else {
        res.json({ success: false, message: 'Invalid file type' });
    }
});

// Start server on port 3000

Need a hint?

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