## Uploading a Large File to a Node Server: A Gentle Introduction ## written by Abdus on 26 September, 2022

Table of Contents

If you’d like to upload large files to a server, but each file is too large to upload as one huge chunk of data. This blog is going to show you how to split your file into smaller chunks and upload them in parallel using Node.js, Express and Multer.

🚀 If you wish to see the code directly, visit this GitHub repository. It is a simple Express code-base, with self-explanatory code. Otherwise, keep reading. I will explain everything in detail.

Project Set-up

Let us start by creating a project. I will go with Express as this is a simple project and does not do much other than just uploading a file! In a production-grade project, I would choose Fastify.

For the front-end, it’s all just HTML and JavaScript.

Create a new directory called file-upload. Initialize a Node project inside it. After that, install express, multer and mime-types (to get file extensions based on the file MIME Type)

mkdir file-upload
cd file-upload

yarn init --yes # choose the default answer for questions
yarn add express multer mime-types

Initialize the Server

Since this is an Express project, we need to write some code to make sure our server runs. Let’s start by importing express and multer into the scope.

import fs from "fs"; // FS IO Operations
import path from "path"; // makes it easy to work with paths
import multer from "multer"; // handles multipart file upload
import express from "express"; // the HTTP server
import mimeTypes from "mime-types"; // returns extension based on a mime-type

Now, let’s set up multer with express. multer is a great library to handle file uploads through a HTML5 Form encoded in multipart format.

const server = express(); // the server
const upload = multer({ dest: "upload/" }); // multer middleware to be used in express routes

server.use(express.json({ limit: "100mb" })); // parse JSON Content-Type
server.use(express.urlencoded({ limit: "100mb", extended: true })); // parse x-www-url-encoded Content-Type

server.use("/", express.static(path.resolve("static"))); // our client-side code lives inside `static` directory

Splitting Files

That’s enough of back-end code for now. Let’s see how we are going to split a user-uploaded file the on client-side.

To accept a file from the browser, I am going to use an input field inside a form element (so that I could take advantage of the submit event).

Once a file is selected, and the user submits the form, a function called chunkify is executed. This function does all the heavy lifting of splitting the file into multiple chunks.

The Blob.slice method helps us split a File into multiple chunks of arbitrary size. File extends Blob, so we can use Blob’s method without any issues.

const ONE_KILO_BYTE = 1024;

function chunkify(file) {
  const chunks = []; // array to store chunks (in order)
  const chunkSize = ONE_KILO_BYTE * 1024; // size of a chunk in bytes. Here, 1MB

  let start = 0;
  let end = chunkSize;

  while (start <= file.size) {
    chunks.push(file.slice(start, end, file.type));

    start = end;
    end += chunkSize;

  return chunks;

Read about Blob.slice on MDN

Okay. So, chunkify will return an array of Blob. Now, we could simply send this Blobs to our server using fetch API and FormData.

chunks.forEach(async (chunk, i) => {
  const formData = new FormData();

  formData.append("file", chunk);
  formData.append("fileName", i);
  formData.append("mimeType", chunk.type);

  const raw = await fetch("/api/upload", {
    body: formData,
    method: "POST",

  const json = await raw.json();

Grouping File Chunks

As of now, the server has no way to know how to group those chunks and merge into a single file. For that, we have to group these chunks somehow.

Going with the easiest way (which may not be an optimal solution), we will give a unique identifier to each group, and add that identifier to each chunk.

To generate a UUID in the browser, we could use crypto.randomUUID(). Here’s how this will work:

const groupId = crypto.randomUUID();

chunks.forEach(async (chunk, i) => {
  const formData = new FormData();

  // ... other file attributes
  formData.append("fileId", groupId); // this will do the trick

  const raw = await fetch("/api/upload", {
    // req options

We also need to let the server know the number of chunks a group will have. For that, we could add another field to the FormData object.

formData.append("chunkCount", chunks.length);

That’s it. Now, time to handle these file chunks on the server.

Handling Files on Server

For Multer to save a file in the specified storage destination, we need to call a middleware, upload.single("name of the field").

server.post("/api/upload", upload.single("file"), (req, res) => {
  try {
    // handle file-upload request
  } catch (err) {
      .json({ msg: "something went wrong", data: err.stack || err.message });

Based on the fields passed into FormData, we can determine the file group along with the type of file. We will create a directory using the file group identifier, and move all the chunks into that group.

const fileId = req.body.fileId;
const uploadedFileName = req.file.path; // get the chunk location (uploaded by multer)
const destDir = path.join("upload", fileId); // chunk storage location

if (!fs.existsSync(destDir)) fs.mkdirSync(destDir, { recursive: true });
if (fs.statSync(destDir).isFile()) {
  fs.mkdirSync(destDir, { recursive: true });

const mimetype = req.body.mimeType; // based on this value, we will get file extension
const fileName = path.join(destDir, req.body.fileName); chunk file-path
const fileExtension = mimeTypes.extension(mimetype);

// move file to the specified folder so that we could merge chunks
// into a single file later
fs.renameSync(uploadedFileName, fileName);

Now that we have uploaded all the chunks to the server, it’s time to merge them into a single file. I have written a function for that.

function mergeFile(dest, extName) {
  // merge needs to be done in order
  const files = fs
    .sort((a, b) => (parseInt(a) > parseInt(b) ? 1 : -1));

  for (let i = 0; i < files.length; i++) {
    const chunkPath = path.join(dest, files[i]);
    const chunkContent = fs.readFileSync(chunkPath);

    fs.appendFileSync(path.join(dest, "merged." + extName), chunkContent); // merging...
    fs.unlinkSync(chunkPath); // delete the chunk. save storage

  // finally return the merged file location
  return path.resolve(path.join(dest, "merged." + extName));

This function accepts two arguments: dest and extName. dest is where the chunk group is located. extName is the merged file extension.

We have to figure out one more thing: when should this file merge happen? One way is to compare the total number of chunks sent from the client and received on the server. Remember, we were sending the chunkCount from the client-side? Time to use it!

I am going to check if the server has all chunks. If so, I will merge. Otherwise, I will wait for the rest of the chunks to arrive at the server.

const chunksRcvd = fs.readdirSync(destDir).length;
const totalChunkCount = parseInt(req.body.chunkCount);

if (chunksRcvd === totalChunkCount) {
  const mergedFilePath = mergeFile(destDir, fileExtension);
  return res.json({
    msg: "success. merged into single file",
    data: mergedFilePath,

return res.json({ msg: "success. awaiting merge", data: null });

That’s it. We are done!


This approach will benefit you in multiple ways, including:

  1. Lesser memory usage. Since, we are not loading the entire file in the RAM, there will be a very small memory footprint.
  2. Faster upload time. Chunks will be uploaded in parallel.
  3. Bypassing web-server restrictions. Web servers, such as Nginx, impose a limit on the HTTP request size.
  4. There may not be lots of performance gain (as heavy IO operations will be performed). But compared to processing a large file, there are performance gains.

Note that we have not utilised all cores of the server. Doing so will boost performance.

I know that this is probably not the optimal solution you were expecting. I will leave that up to you to modify it according to your needs.