JavaScript asynchronously uploading files

At the beginning I have thought, that it has to be easy; just make a POST to the server using jQuery and the only question is how to get the data. Well I have found out that it is not that easy and googling around I have found there are quite a lot of pre-build components and plugins, which makes it quite difficult to decide for one of those.

Why it is not possible to use simple JavaScript POST?

Because of the security restrictions. The browser is not allowed to post the file content asynchronously. This is however about to change thanks to HTML 5.


  • HTML 5 - has a support for file uploading. Use the File API. Follow this how to. This does not work in current versions of IE (7,8,9).
  • Create a hidden iFrame on the page and redirect the return of the post to this iFrame
  • Use Flash, Silverlight, or Java applet
  • Use some component, or jQuery plugin, which usually makes use of the preceding ones (usually the iFrame hack)

jQuery plugins

There are quite few of those:

I have tested jQuery File Upload. Which is cool, comes with nice GUI but at the time of writing this, I have found it little hard to customize. Actually I have struggled to use a simple form, which would upload just one file, instead of the predefined GUI with it’s behavior.

The second one that I have tested is jQuery Form Plugin which contrary to the previous one, is simple to use in a one file upload scenario. However it does not provide the nice UI, ready for multiple files upload etc…

Using jQuery Form Plugin in ASP.NET

Client side

On the client side you need jQuery and the Plugin js file. Then with one jQuery call you can set up the form, to use the plugin.

<form id="uploadForm" action="Upload.ashx" method="POST" enctype="multipart/form-data">
  <input type="hidden" name="MAX_FILE_SIZE" value="100000" />
  <input type="file" name="file" />
  <input type="submit" value="Submit" />
    beforeSubmit: function (a, f, o) {
        o.dataType = 'html'
    success: function (data) {
        alert('upload OK:' + data);

The dataType property which is set to ‘html, specifies to Form Plugin what kind of response should it expect. To check the other options see the documentation.

You can see, that the form action is set to “Upload.ashx”. This is the server side script, being a Http Handler (in case of ASP.NET application). It could also probably be a WCF service - but let’s keep it simple when we can.

Server side

On the server side you have to define a Http Handler which will take care of the upload functionality.

public class Upload : IHttpHandler
    public void ProcessRequest(HttpContext context)
        System.Web.HttpPostedFile file = context.Request.Files[0];
        String filePath = "Uploads" + "\\" + RandomString(10) + "." + extension;
        string linkFile = System.Web.HttpContext.Current.Server.MapPath("~") + filePath;

        context.Response.StatusCode = 200;

And that’s it. The handler will save the file and send back the address of the file.

Written on February 18, 2012