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What is the difference between multer's diskStorage and memoryStorage engines? Why should an application typically avoid storing uploaded files directly on the same server instance that runs the web application? How would you validate and restrict uploaded file types and sizes using multer to prevent malicious or oversized uploads? How would you stream a large file upload directly to processing (like virus scanning or transcoding) without first saving the entire file to disk? Why should CPU-intensive media processing like image resizing or video transcoding typically not run directly inside an Express request handler? How would you validate an uploaded image's actual dimensions and reject ones that don't meet minimum or maximum size requirements? How would you implement a progress indicator for a large file upload from the client's perspective, given the Node.js server side? What is the difference between upload.single(), upload.array(), and upload.fields() in multer? How would you process and resize an uploaded image in Node.js using the sharp library, and why is sharp generally preferred over pure-JavaScript alternatives? How would you implement direct-to-S3 file uploads from a client, avoiding routing large file uploads through your Node.js server entirely?
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Node.js › File Uploads & Media Processing

What is multipart/form-data, and why does a Node.js server need special middleware (like multer) to handle file uploads?

Beginner

Answer

multipart/form-data is the HTTP content type browsers use to submit forms containing file uploads, encoding each form field (including file contents) as a separate 'part' within the request body, separated by a boundary string -- Express's built-in body-parsing middleware (express.json/urlencoded) doesn't understand this format, so a dedicated middleware like multer is needed to parse the multipart body, extract uploaded files (saving them to disk or memory), and populate req.file/req.files with the parsed results.
const multer = require('multer');
const upload = multer({ dest: 'uploads/' });

app.post('/upload', upload.single('avatar'), (req, res) => {
  console.log(req.file); // { filename, path, size, mimetype, ... }
  res.json({ success: true });
});
Real-world example A profile-picture upload endpoint uses multer configured to save uploaded files to a temporary directory, then processes the resulting req.file to move it to permanent storage after validating its size and type.

Common follow-ups: Why can't express.json() parse a multipart/form-data request the way it parses a JSON body?;What's the difference between multer's disk storage and memory storage engines?

HTTP & Web Servers;Express & Middleware
All 15 File Uploads & Media Processing questions