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What is multipart/form-data, and why does a Node.js server need special middleware (like multer) to handle file uploads? 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 the difference between multer's diskStorage and memoryStorage engines?

Beginner

Answer

diskStorage writes uploaded files directly to a specified directory on disk as they're received, returning a file path in req.file, suitable for larger files where holding the entire content in memory would be wasteful. memoryStorage keeps the uploaded file entirely in a Buffer in memory (available as req.file.buffer), convenient for smaller files that will be immediately processed or forwarded elsewhere (like directly to cloud storage) without ever needing to touch the local disk.
// Disk storage: writes to disk, file path available
const upload = multer({ storage: multer.diskStorage({ destination: 'uploads/' }) });

// Memory storage: buffer available directly, no disk write
const upload = multer({ storage: multer.memoryStorage() });
app.post('/upload', upload.single('file'), (req, res) => {
  s3.putObject({ Body: req.file.buffer, Key: req.file.originalname });
});
Real-world example An avatar-upload endpoint uses memoryStorage since uploaded images are small and immediately forwarded to S3 without ever needing a local disk copy, while a large-video-upload endpoint uses diskStorage to avoid holding potentially gigabyte-sized files entirely in memory.

Common follow-ups: What's the memory risk of using memoryStorage for an endpoint that might receive very large files?;How would you clean up temporary files left on disk by diskStorage after they've been processed?

Streams & Buffers;Cloud & DevOps
All 15 File Uploads & Media Processing questions