Handling File Uploads in Fastify Using Multipart

Streaming file uploads in Fastify with fastify-multipart — no buffering everything into memory.

Prerequisites

• Node.js >=16
• Fastify project setup

Step 1: Install the Plugin

npm install fastify-multipart

Step 2: Register Plugin

In src/server.ts:

import Fastify from "fastify";
import multipart from "fastify-multipart";

const app = Fastify({ logger: true });
app.register(multipart); 

app.post("/upload", async (req, reply) => {
  const parts = req.parts();
  for await (const part of parts) {
    if (part.file) {
      await pump(part.file, fs.createWriteStream(`uploads/${part.filename}`));
    }
  }
  reply.send({ status: "ok" });
});

app.listen({ port: 3000 });

Step 3: Handle Streams and Save Files

Install utility:

npm install pump

Use pump to pipe the file stream:

import { pump } from "pump";
import fs from "fs";

Step 4: Test Uploads

curl -F "file=@./path/to/file.png" http://localhost:3000/upload

Streaming multipart means large files won’t eat your RAM. Register the plugin, iterate req.parts(), pipe each file stream to disk or S3.