Eli Grey

Saving generated files on the client-side

Have you ever wanted to add a Save as… button to a webapp? Whether you’re making an advanced WebGL-powered CAD webapp and want to save 3D object files or you just want to save plain text files in a simple Markdown text editor, saving files in the browser has always been a tricky business.

Usually when you want to save a file generated with JavaScript, you have to send the data to your server and then return the data right back with a Content-disposition: attachment header. This is less than ideal for webapps that need to work offline. The W3C File API includes a FileSaver interface, which makes saving generated data as easy as saveAs(data, filename), though unfortunately it will eventually be removed from the spec.

I have written a JavaScript library called FileSaver.js, which implements FileSaver in all modern browsers. Now that it’s possible to generate any type of file you want right in the browser, document editors can have an instant save button that doesn’t rely on an online connection. When paired with the standard HTML5 canvas.toBlob() method, FileSaver.js lets you save canvases instantly and give them filenames, which is very useful for HTML5 image editing webapps. For browsers that don’t yet support canvas.toBlob(), Devin Samarin and I wrote canvas-toBlob.js. Saving a canvas is as simple as running the following code:

canvas.toBlob(function(blob) {
    saveAs(blob, filename);

I have created a demo of FileSaver.js in action that demonstrates saving a canvas doodle, plain text, and rich text. Please note that saving with custom filenames is only supported in browsers that either natively support FileSaver or browsers like Google Chrome 14 dev and Google Chrome Canary, that support <a>.download or web filesystems via LocalFileSystem.

How to construct files for saving

First off, you want to instantiate a Blob. The Blob API isn’t supported in all current browsers, so I made Blob.js which implements it. The following example illustrates how to save an XHTML document with saveAs().

      new Blob(
          [(new XMLSerializer).serializeToString(document)]
        , {type: "application/xhtml+xml;charset=" + document.characterSet}
    , "document.xhtml"

Not saving textual data? You can save multiple binary Blobs and ArrayBuffers to a Blob as well! The following is an example of setting generating some binary data and saving it.

      buffer = new ArrayBuffer(8) // allocates 8 bytes
    , data = new DataView(buffer)
// You can write uint8/16/32s and float32/64s to dataviews
data.setUint8 (0, 0x01);
data.setUint16(1, 0x2345);
data.setUint32(3, 0x6789ABCD);
data.setUint8 (7, 0xEF);
saveAs(new Blob([buffer], {type: "example/binary"}), "data.dat");
// The contents of data.dat are <01 23 45 67 89 AB CD EF>

If you’re generating large files, you can implement an abort button that aborts the FileSaver.

var filesaver = saveAs(blob, "video.webm");
abort_button.addEventListener("click", function() {
}, false);


mumbl is a JavaScript library that makes it easy to play music and create playlists on web pages.


A demo is included with mumbl but if you dont want to download it, there is also an online demo.

Please note that mumbl is not the player in the demo. mumbl is the back-end and the demo is just an example of using mumbl.

Supported Platforms

Supported Platforms

  • HTML5
    • Firefox 3.5+
    • Google Chrome 4+
  • SoundManager 2 (version 2.95b.20100323+)
    • Firefox 1.5+
    • Opera 10+
    • Google Chrome 1+
  • Songbird 1.4+


API documentation can be found in the readme.


  • 0.1.1
    • Better error handling.
    • loaderror event.
  • A while after version 0.1 is released
    • Create a simple library that makes all MP3, OGG, WAV, etc. links be able to be played using mumbl.
    • Make the demo mumbl-powered music player (it will be renamed “mumblr”) portable and reusable.
    • Remove jQuery dependency from mumblr.
    • Make the track title display scroll (maybe using a <marquee>) when it overflows.
  • Version 0.2
    • Full compatability with every major browser.
  • The distant future (maybe version 1.0)
    • Create a simplified flash audio back-end for mumbl that integrates much more nicely and has a smaller file size than SoundManager2.

pmxdr: postMessage cross-domain request library

pmxdr is a cross-domain HTTP request JavaScript library. pmxdr stands for postMessage cross-domain requester. As the name implies, it makes use of the HTML5 postMessage API to make HTTP requests. It requires that a pmxdr host be on the target domain and it respects all HTTP access control headers, even on browsers that don’t support them but do support postMessage, like Firefox 3.

You can download the pmxdr client library and the pmxdr host library (includes an example .htaccess file to help Apache users with PHP set it up) under the latest GNU GPL license and an MIT-style license. The host library must be able to be accessed from /pmxdr/api to be able to interact with the client library.

Read more at the pmxdr project page and try out the demo of it in action.

The following is a very simple example of how to use pmxdr to do a cross-domain POST request (impossible with the normal method of inserting a script tag):

// The requesting domain is example.net
// example.net doesn't want to give any control to example.com
  method   : "post",
  uri      : "http://example.com/search.json",
  data     : "q=foo",
  callback : loadSearchJSON

Usually, in this hypothetical example, example.net would have to put a script tag with an src of http://example.com/search.json?q=foo&callback=loadSearchJSON and give example.com full control of example.net. This provides a secure cross-domain way to use APIs like this.

HTML 5 dataset support

Update: Getting (not setting) data-* attributes through Element.dataset now works in IE8 with the help of Xccessors.

I have made an implementation of HTML 5 dataset (data-* attributes) support that almost conforms to the HTML 5 spec* and works in Firefox 1.5+, Opera 9.5+, Safari, and Google Chrome. It uses getters and setters to simulate a psuedo-native HTML 5 dataset API. The code is licensed CC GNU LGPL and can be downloaded here. I have uploaded a demo (also in XHTML 5) that you can try out. The script includes these extra functions: Element.removeDataAttribute(name), Element.setDataAttribute(name, value), and Element.setDataAttributes() (all explained below).

*You cannot set new items the standard way like Element.dataset.name = value (already existing items can though). You either have to add new items by doing Element.setDataAttribute(name, value) for single additions and Element.setDataAttributes(object full of {name, value:String} pairs) to add multiple items at once. Instead of delete element.dataset[name], you must use element.dataset[name] = undefined or Element.removeDataAttribute(name) to remove data-name. Example of setting values:

var foo = document.createElement('div');
foo.setDataAttributes({'bar':'blah', 'lorem':'Lorem ipsum.'});
foo.dataset.bar == 'blah';
foo.dataset.bar = 'eligrey.com';
foo.dataset.bar == 'eligrey.com';
foo.setDataAttribute('foobar', foo.dataset.lorem);
foo.dataset.foobar == 'Lorem ipsum.'