Mikael's blog

A developers seventh time trying to maintain a blog

Async File Uploads in HTML5

Uploading files using HTML forms has always felt a bit off for me. You had to set your encoding to multipart/form-data and the synchronous nature of form posts always made it a waiting game when uploading larger files.

Then came AJAX and the dawn of “single page applications” and such buzzwords, but file uploads somehow got left behind. Javascript didn’t have the access it needed to send files with the XMLHttpRequest. Years passed and along came the File API and XMLHttpRequest Level 2 (it seems to be called that again) with its upload attribute, support for byte streams and progress events.

Today I’m going to show you how to build an asynchronous file uploader with it.

We’ll start with the HTML part:

<input type="file" id="files-upload" multiple />

<ul id="file-list"></ul>

There’s nothing weird going on here; just a regular file selector and a list of uploaded files. The multiple attribute is added so that we can upload more than one file at the time.

We want our files to upload as soon as they are selected so we hook up to the change event on our file selector input.

var filesUpload = document.getElementById('files-upload');
filesUpload.addEventListener("change", function () {
}, false);

The traverseFiles function checks for File API support and calls the upload function on each of the selected files:

function traverseFiles (files) {
  if (typeof(files) !== "undefined") {
    for (var i = 0; i < files.length; i++) {
  else {
    var fileList = document.getElementById('file-list');
    fileList.innerHTML = "No support for the File API in this web browser";


The upload function is where the actual uploading happens:

function upload(file) {
  var li = document.createElement("li"),
    div = document.createElement("div"),
    progressBarContainer = document.createElement("div"),
    progressBar = document.createElement("div"),
    fileList = document.getElementById('file-list'),

  progressBarContainer.className = "progress-bar-container";
  progressBar.className = "progress-bar";

  // add list item with progress bar

  // create the XMLHttpRequest object
  xhr = new XMLHttpRequest();
  // attach progress bar event
  xhr.upload.addEventListener("progress", function (e) {
    if (e.lengthComputable) {
      progressBar.style.width = (e.loaded / e.total) * 100 + "%";
  }, false);
  // attach ready state change event
  xhr.addEventListener("readystatechange", function () {
    if (xhr.readyState == 4) { // if we are done
      if (xhr.status == 200) { // and if we succeded
        progressBarContainer.className += " uploaded";
        progressBar.innerHTML = "Uploaded!";
        div.appendChild(document.createTextNode(file.name + '(' + file.size + ')'));
      } else {
        div.className += ' upload-error';
        progressBar.innerHTML = "Failed!";
  }, false);

  // open the request asynchronously
  xhr.open("post", "/upload", true);
  // set some request headers
  xhr.setRequestHeader("Content-Type", "multipart/form-data");
  xhr.setRequestHeader("X-File-Name", file.name);
  xhr.setRequestHeader("X-File-Size", file.size);
  xhr.setRequestHeader("X-File-Type", file.type);

  // send the file

Note how we bind to the progress event on xhr.upload. That is because the upload property is actually another type of XMLHttpRequest object that is similar but not the same as its parent. In this case, all you need to now is that all the events for the actual upload propagates on this object.

Receiving the file

The node.js side of things is not that hard either, assuming that the server below is forwarded to from the /upload URL that we posted to.

var http = require('http');
var fs = require('fs');

function onRequest(request, response) {

  var postData = '';
  request.addListener('data', function (postDataChunk) {
    postData += postDataChunk;

  request.addListener('end', function () {
    var fileName = request.headers['x-file-name'];

    fs.writeFile(fileName, postData, { encoding: 'binary' }, function(err) {
      if (err) {
        response.write("File write error.");
      } else {
        response.writeHead(200, 'application/json');


The important thing to note here is the request.setEncoding('binary');, at the beginning of the onRequest method, and the {encoding: 'binary'} option sent to fs.writeFile().

Of course, in a real world situation you might want some more logic in place to ward of misuse from malicious people or control what type of files get uploaded. In my case I added some logic for making sure (it’s in the admin section of my blog so I don’t have to be 100% sure) that I only upload images.

I also do some nifty re-sizing of my images before they are uploaded which takes care of long upload times and default size limits for XMLHttpRequests (Chrome defaults to 1 MB) but that is the subject for another time.

by Mikael Lofjärd
I'm sorry, but comments are not implemented yet.

Sorry, sharing is not available as a feature in your browser.

You can share the link to the page if you want!