File Upload Service
Part of the Backend Developer Roadmap
In short
A file upload service is a small backend that only does one job: accept files safely, store them, and give them back. It's a perfect project to learn limits, streams and security.
A dedicated service for accepting, validating and storing file uploads — practice for Multer, file validation, and cloud storage integration.
Think of it like this
The railway cloakroom
At the station cloakroom, you hand over your bag, it's checked and shelved, and you get a token. Later, show the token and get your bag back. Your service does the same: take the file, store it, return an id to fetch it later.
Watch it happen
Upload, then download
POST /files (multipart)
1/6The client uploads invoice.pdf in a field called file.
The big ideas
What you need to know
Limits first
Max size, allowed types and maybe max files per user per day. Decide these before writing any code.
Metadata separate
Store the file in storage and its details in a database. Listing files never touches the disk.
Private by default
Downloads check who's asking. Never serve the uploads folder with express.static.
Stream downloads
res.download or a read stream sends big files in chunks, so memory stays low.
See it in code
Walk through the code
1app.get("/files/:id", requireAuth, async (req, res, next) => {2 const meta = await db.files.findById(req.params.id);3 if (!meta || meta.ownerId !== req.user.id) {4 return res.status(404).json({ error: "File not found" });5 }6 7 const filePath = path.join(UPLOAD_DIR, meta.storedName);8 res.download(filePath, meta.originalName, (err) => {9 if (err && !res.headersSent) next(err);10 });11});1/4Only logged-in users can download. Look up the file's metadata by id.
Returning 404 instead of 403 for someone else's file means nobody can tell whether a guessed id exists.
Save yourself the pain
Common mistakes
Oops
Building the file path from req.params directly, like path.join(dir, req.params.name).
Do this instead
Look up the stored name in your database. User input in paths allows ../ tricks to read other files.
Oops
Reading the whole file into memory with fs.readFile before sending it.
Do this instead
Stream it with res.download or fs.createReadStream so large files don't eat your RAM.
Your mission
Milestone 1: upload and fetch
Quick check
Why store an uploaded file under a random name instead of its original filename?
Go deeper
- 1Accept uploads with Multer and validate file type/size before saving.
- 2Upload files to cloud storage (S3 or equivalent) instead of local disk.
- 3Return a public or signed URL for each uploaded file.
More projects at this stop
REST API for Todo App
Blog API
Stuck on this topic? Ask an Insider
Get 1:1 guidance from people who've walked this exact path — on the InsideEdge app.
