**Course Title:** Mastering Express.js: Building Scalable Web Applications and APIs **Section Title:** File Uploads and Handling Form Data **Topic:** Handling multipart/form-data **Introduction:** In this topic, we will explore the concept of handling multipart/form-data in Express.js applications. Multipart/form-data is a type of HTTP request that allows users to upload multiple files to a server. This is commonly used in web applications that require file uploads, such as image galleries, file sharing platforms, and more. **Key Concepts:** * Multipart/form-data HTTP request * Express.js middleware for handling file uploads * Multer middleware for handling multipart/form-data requests * File upload validation and storage * Handling file uploads in Express.js routes **Understanding Multipart/form-data:** Multipart/form-data is a type of HTTP request that allows users to upload multiple files to a server. This type of request is used when a user wants to upload multiple files, such as images, documents, and videos. The request is formatted as a series of key-value pairs, where each key-value pair represents a file to be uploaded. For example, a user might upload two files, an image and a document, using the following request: ```http POST /upload HTTP/1.1 Content-Type: multipart/form-data; boundary=---------------------------1234567890 ---------------------------1234567890 Content-Disposition: form-data; name="file1"; filename="image.jpg" Content-Type: image/jpeg ...image data... ---------------------------1234567890 Content-Disposition: form-data; name="file2"; filename="document.pdf" Content-Type: application/pdf ...document data... ``` **Express.js Middleware for Handling File Uploads:** Express.js provides a middleware function called `multer` that can be used to handle multipart/form-data requests. Multer is a popular middleware for handling file uploads in Express.js applications. To use Multer, you need to install it as a dependency in your project: ```bash npm install multer ``` Then, you can use Multer in your Express.js application like this: ```javascript const express = require('express'); const multer = require('multer'); const app = express(); const upload = multer({ dest: './uploads/', // directory for uploaded files limits: { fileSize: 1024 * 1024 * 5 } // 5MB file size limit }); app.post('/upload', upload.single('file'), (req, res) => { // req.file contains the uploaded file // req.body contains the entire request body console.log(req.file); console.log(req.body); res.send('File uploaded successfully!'); }); ``` **Multer Middleware for Handling Multipart/form-data Requests:** Multer provides several options for handling multipart/form-data requests. Some of the most common options include: * `dest`: specifies the directory for uploaded files * `limits`: specifies the maximum file size and the maximum number of files to be uploaded * `fileFilter`: specifies a function that can be used to filter out files that do not meet certain criteria For example, you can use the following code to upload files up to 5MB in size and store them in the `./uploads/` directory: ```javascript const upload = multer({ dest: './uploads/', limits: { fileSize: 1024 * 1024 * 5 } }); ``` **File Upload Validation and Storage:** When handling file uploads, it's essential to validate the uploaded files to ensure they meet certain criteria. Some common validation criteria include: * File type: only allow certain types of files to be uploaded * File size: only allow files of a certain size to be uploaded * File extension: only allow files with certain extensions to be uploaded You can use the `multer` middleware to validate files based on these criteria. For example, you can use the following code to only allow image files to be uploaded: ```javascript const upload = multer({ dest: './uploads/', limits: { fileSize: 1024 * 1024 * 5 }, fileFilter: (req, file, cb) => { if (!file.originalname.match(/\.(jpg|jpeg|png)$/)) { return cb(new Error('Only image files are allowed!')); } cb(null, true); } }); ``` **Handling File Uploads in Express.js Routes:** Once you've set up the Multer middleware, you can use it in your Express.js routes to handle file uploads. For example, you can use the following code to upload a file and store it in the `./uploads/` directory: ```javascript app.post('/upload', upload.single('file'), (req, res) => { // req.file contains the uploaded file // req.body contains the entire request body console.log(req.file); console.log(req.body); res.send('File uploaded successfully!'); }); ``` **Conclusion:** In this topic, we've covered the basics of handling multipart/form-data in Express.js applications. We've discussed the key concepts of multipart/form-data, Express.js middleware for handling file uploads, and Multer middleware for handling multipart/form-data requests. We've also covered file upload validation and storage, and how to handle file uploads in Express.js routes. **Practical Takeaways:** * Use Multer middleware to handle multipart/form-data requests in your Express.js applications. * Validate uploaded files based on certain criteria, such as file type, file size, and file extension. * Store uploaded files in a directory, such as the `./uploads/` directory. * Use the `multer` middleware to handle file uploads in your Express.js routes. **Additional Resources:** * [Multer documentation](https://github.com/expressjs/multer#multer) * [Express.js middleware documentation](https://expressjs.com/en/guide/middleware.html) **Leave a comment or ask for help:**