Today, we'll learn how to upload a file from the frontend client to be downloaded in the backend server.

The frontend will use React Next.JS and the backend will use Node.JS.

Create your frontend and backend projects

See A simple React Next + Node app in Typescript.

Create input in frontend

First, install axios in frontend/. We're using axios instead of the native fetch to post the file since we want to print the upload progress.

npm i axios

In frontend/src/app/page.tsx, let's create a very basic file input:

"use client";
import { useEffect, useState } from "react";
import axios from "axios";

export default function Home() {
  async function handleUploadFile(file?: File) {
    if (!file) return;
    const formData = new FormData();
    formData.append("file", file);
    const res = await axios.post("http://localhost:8000/uploadFile", formData, {
      onUploadProgress: (progressEvent) => {
        console.log(
          "Upload progress: " +
            Math.round(
              (progressEvent.loaded / (progressEvent.total ?? 1)) * 100
            ) +
            "%"
        );
      },
    });
    console.log(res);
  }

  return (
    <div>
      <form>
        <input
          type="file"
          name="name"
          onChange={(e) =>
            handleUploadFile((e.target as HTMLInputElement).files?.[0])
          }
        />
      </form>
    </div>
  );
}

Hence, http://localhost:3000 will look like:

Set up multer in backend

Multer is the service we'll use as the middleware to store the received file in the hard disk.

Let us install multer first:

npm i multer @types/multer

Then, modify backend/index.tsx to the following:

import express from "express";
import cors from "cors";
import multer from "multer";


// Create express app
const app = express();

app.use(express.json());
app.use(cors());

app.get("/helloText", (req, res) => {
  res.send({ myResponse: "Hello World!" });
});

const upload = multer({ dest: "tmp/uploads/" });
app.post("/uploadFile", upload.single("file"), (req, res) => {
  console.log(req.file);
  res.send({ myResponse: "File uploaded successfully" });
});


app.listen(8000, () => {
  console.log(`Server is running on port ${8000}`);
});

We are adding the /uploadFile POST path where we will receive and download the file to store in directory tmp/uploads/.

Upload and verify

I have this simple file called a.json which contains a very simple text:

{
    "message": "Hello"
}

On localhost:3000, I am uploading this file through the input box. I am also opening the console log to see the upload progress:

And in the backend, I see a new file with a random name in backend/tmp/uploads/, which is a copy of the file I just uploaded:

Of course, this works for other types of files up to the size limit imposed by your browser and internet speed.

That's all, good luck!

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