File upload & download handling

Let’s go through file upload and download handling in Node.js, which is essential for APIs that deal with user files, images, or documents. We’ll cover it using Express.

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1️⃣ File Upload Handling

a) Using multer

• multer = middleware for handling multipart/form-data, which is the encoding used for file uploads.

Install

npm install multer

b) Basic Setup

const express = require("express");
const multer = require("multer");
const path = require("path");
 
const app = express();
 
// Storage configuration
const storage = multer.diskStorage({
  destination: (req, file, cb) => {
    cb(null, "uploads/"); // folder to save files
  },
  filename: (req, file, cb) => {
    cb(null, Date.now() + path.extname(file.originalname)); // unique filename
  }
});
 
const upload = multer({ storage });
 
// Single file upload
app.post("/upload", upload.single("file"), (req, res) => {
  res.json({ message: "File uploaded", file: req.file });
});
 
// Multiple files upload
app.post("/upload-multiple", upload.array("files", 5), (req, res) => {
  res.json({ message: "Files uploaded", files: req.files });
});
 
app.listen(3000, () => console.log("Server running on port 3000"));

Notes:

• req.file → for single file

• req.files → for multiple files

• Always validate file type and size

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c) File Type & Size Validation

const upload = multer({
  storage,
  limits: { fileSize: 5 * 1024 * 1024 }, // 5MB
  fileFilter: (req, file, cb) => {
    const allowedTypes = /jpeg|jpg|png|pdf/;
    const extname = allowedTypes.test(path.extname(file.originalname).toLowerCase());
    if (extname) return cb(null, true);
    cb(new Error("Only images or PDFs are allowed"));
  }
});

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2️⃣ File Download Handling

• Use res.download() or res.sendFile() in Express

app.get("/download/:filename", (req, res) => {
  const filePath = path.join(__dirname, "uploads", req.params.filename);
  res.download(filePath, (err) => {
    if (err) res.status(404).json({ error: "File not found" });
  });
});

• res.download() → triggers file download prompt in browser

• res.sendFile() → serves file inline

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3️⃣ Streaming Large Files

• For large files, use streams instead of reading the entire file into memory:

const fs = require("fs");
 
app.get("/stream/:filename", (req, res) => {
  const filePath = path.join(__dirname, "uploads", req.params.filename);
  const readStream = fs.createReadStream(filePath);
  
  readStream.on("error", () => res.status(404).json({ error: "File not found" }));
  readStream.pipe(res); // stream file directly to response
});

• Efficient for large media files and video/audio streaming

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4️⃣ Security Considerations

1. Validate file type and size → prevent malicious files

2. Store files outside of the public root → avoid direct access

3. Generate unique filenames → prevent collisions and overwrites

4. Scan uploaded files (optional) → virus/malware scanning

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✅ Summary

Operation	Method / Library	Notes
Upload	multer	Single/multiple files, validation, disk/memory storage
Download	res.download / res.sendFile	Trigger download or inline serve
Streaming	fs.createReadStream()	Efficient for large files
Security	Validate type/size, store safely	Prevent malicious access

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