April 23, 2012


It's a JavaScript library providing multiple simultaneous, stable and resumable uploads via the HTML5 File API.

The library is designed to introduce fault-tolerance into the upload of large files through HTTP. This is done by splitting each files into small chunks; whenever the upload of a chunk fails, uploading is retried until the procedure completes. This allows uploads to automatically resume uploading after a network connection is lost either locally or to the server. Additionally, it allows for users to pause, resume and even recover uploads without losing state.

Resumable.js does not have any external dependencies other the HTML5 File API. This is relied on for the ability to chunk files into smaller pieces. Currently, this means that support is limited to Firefox 4+ and Chrome 11+.

Samples and examples are available in the samples/ folder. Please push your own as Markdown to help document the project.

How can I use it?

A new Resumable object is created with information of what and where to post:

var r = new Resumable({
// Resumable.js isn't supported, fall back on a different method
if(!r.support) location.href = '/some-old-crappy-uploader';

To allow files to be either selected and drag-dropped, you'll assign drop target and a DOM item to be clicked for browsing:


After this, interaction with Resumable.js is done by listening to events:

r.on('fileAdded', function(file){
r.on('fileSuccess', function(file,message){
r.on('fileError', function(file, message){

How do I set it up with my server?

Most of the magic for Resumable.js happens in the user's browser, but files still need to be reassembled from chunks on the server side. This should be a fairly simple task and can be achieved in any web framework or language, which is able to receive file uploads.

To handle the state of upload chunks, a number of extra parameters are sent along with all requests:

  • resumableChunkNumber: The index of the chunk in the current upload. First chunk is 1 (no base-0 counting here).
  • resumableChunkSize: The general chunk size. Using this value and resumableTotalSize you can calculate the total number of chunks. Please note that the size of the data received in the HTTP might be lower than resumableChunkSize of this for the last chunk for a file.
  • resumableTotalSize: The total file size.
  • resumableIdentifier: A unique identifier for the file contained in the request.
  • resumableFilename: The original file name (since a bug in Firefox results in the file name not being transmitted in chunk multipart posts).

You should allow for the same chunk to be uploaded more than once; this isn't standard behaviour, but on an unstable network environment it could happen, and this case is exactly what Resumable.js is designed for.

For every request, you can confirm reception in HTTP status codes:

  • 200: The chunk was accepted and correct. No need to re-upload.
  • 415. 500, 501: The file for which the chunk was uploaded is not supported, cancel the entire upload (in fact, any >=400 HTTP status code will trigger this result, see details.
  • Anything else: Something went wrong, but try reuploading the file.

Handling GET (or test() requests)

This will allow uploads to be resumed after browser restarts and even across browsers (in theory you could even run the same file upload across multiple tabs or different browsers). The POST data requests listed are required to use Resumable.js to receive data, but you can extend support by implementing a corresponding GET request with the same parameters:

  • If this request returns a 200 HTTP code, the chunks is assumed to have been completed.
  • If the request returns anything else, the chunk will be uploaded in the standard fashion.

Full documentation



The object is loaded with a configuation hash:

var r = new Resumable({opt1:'val', ...});

Available configuration options are:

  • target The target URL for the multipart POST request (Default: /)
  • chunkSize The size in bytes of each uploaded chunk of data (Default: 1*1024*1024)
  • simultaneousUploads Number of simultaneous uploads (Default: 3)
  • fileParameterName The name of the multipart POST parameter to use for the file chunk (Default: file)
  • query Extra parameters to include in the multipart POST with data (Default: {})
  • prioritizeFirstAndLastChunk Prioritize first and last chunks of all files. This can be handy if you can determine if a file is valid for your service from only the first or last chunk. For example, photo or video meta data is usually located in the first part of a file, making it easy to test support from only the first chunk. (Default: false)
  • testChunks Make a GET request to the server for each chunks to see if it already exists. If implemented on the server-side, this will allow for upload resumes even after a browser crash or even a computer restart. (Default: true)


  • .support A boolean value indicator whether or not Resumable.js is supported by the current browser.
  • .opts A hash object of the configuration of the Resumable.js instance.
  • .files An array of ResumableFile file objects added by the user (see full docs for this object type below).


  • .assignBrowse(domNodes) Assign a browse action to one or more DOM nodes.
  • .assignDrop(domNodes) Assign one or more DOM nodes as a drop target.
  • .on(event, callback) Listen for event from Resumable.js (see below)
  • .upload() Start or resume uploading.
  • .pause() Pause uploading.
  • .progress() Returns a float between 0 and 1 indicating the current upload progress of all files.
  • .isUploading() Returns a boolean indicating whether or not the instance is currently uploading anything.
  • .removeFile(file) Cancel upload of a specific ResumableFile object on the list from the list.
  • .getFromUniqueIdentifier(uniqueIdentifier) Look up a ResumableFile object by its unique identifier.


  • .fileSuccess(file) A specific file was completed.
  • .fileProgress(file) Uploading progressed for a specific file.
  • .fileAdded(file) A new file was added.
  • .fileRetry(file) Something went wrong during upload of a specific file, uploading is being retried.
  • .fileError(file, message) An error occured during upload of a specific file.
  • .complete() Uploading completed.
  • .progress() Uploading progress.
  • .error(message, file) An error, including fileError, occured.
  • .pause() Uploading was paused.
  • .catchAll(event, ...) Listen to all the events listed above with the same callback function.



  • .resumableObj A back-reference to the parent Resumable object.
  • .file The correlating HTML5 File object.
  • .fileName The name of the file.
  • .size Size in bytes of the file.
  • .uniqueIdentifier A unique identifier assigned to this file object. This value is included in uploads to the server for reference, but can also be used in CSS classes etc when building your upload UI.
  • .chunks An array of ResumableChunk items. You shouldn't need to dig into these.


  • .progress(relative) Returns a float between 0 and 1 indicating the current upload progress of the file. If relative is true, the value is returned relative to all files in the Resumable.js instance.
  • .abort() Abort uploading the file.
  • .cancel() Abort uploading the file and delete it from the list of files to upload.
  • .retry() Retry uploading the file.
  • .bootstrap() Rebuild the state of a ResumableFile object, including reassigning chunks and XMLHttpRequest instances.


This library is explicitly designed for modern browsers supporting advanced HTML5 file features, and the motivation has been to provide stable and resumable support for large files (allowing uploads of several GB files through HTTP in a predictable fashion).

If your aim is just to support progress indications during upload/uploading multiple files at once, Resumable.js isn't for you. In those cases, SWFUpload and Plupload provides the same features with wider browser support.