Modernizr.on() polyfill
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Modernizr.on() prollyfill

Prollyfill: a polyfill for a not yet standardized API.

- Alex Sexton, 2012

A polyfill for the (potentially) forthcoming Modernizr.on() method, for Modernizr 2.x.

Update: Modernizr.on() did indeed make it into Modernizr 3.0 so now it’s best to just use the new builder for new projects. This project is still useful if you’re rocking a 2.x build like some kind of retro superstar and don’t want to deal with 3.x API changes.


Handling asynchronous detects in Modernizr is currently a pain. These are ones which require some deferred action, so won't have finished by the time the rest of Modernizr finishes running:

<script src="modernizr.custom.js"></script>
  console.log(Modernizr.datauri); // undefined

At time of writing, there are 8 detects which suffer this plight: todataurl, webplossless, webp, scriptasync, svgasimg, datauri, blobworkers and dataworkers.

Modernizr v3.0 (currently in development) is expected to include a new method to make these easier to work with, which at present goes by the name of Modernizr.on().

I should make it clear that the details aren't final yet... but it's prolly gonna look like this:

Modernizr.on(property, callback)


  • property is the name of a feature in your Modernizr build (e.g. 'datauri')
  • callback is a function to be called when that test has completed, which takes a single argument: the result of the test (usually a Boolean)

If the test has already completed (including if you attach a callback to a synchronous test), the callback will be called almost immediately (it'll actually be on the next tick).

The polyfill

So, in case you want to start playing with this functionality now (i.e. on 2.x), here's a very crude, fairly inefficient polyfill.


Include the modernizr-on.min.js script after Modernizr, but before any calls to Modernizr.on(). This normally means you can put it with the rest of your scripts at the bottom of your page (if that's how you roll):

<!DOCTYPE html>
  <script src="modernizr.custom.js"></script>
  <!-- HTML things -->
  <script src="modernizr-on.min.js"></script>
  <script src="your-other-scripts.js"></script>

Then you can use it in your scripts something like this:

Modernizr.on('datauri', function (result) {
  if (result) {
    // Browser supports data URIs
  else {
    // Browser doesn't support data URIs

Please don't call Modernizr.on() with a property name which doesn't exist in your Modernizr build, because it'll keep looking for it forever and waste CPU cycles. Like I said, it's pretty crude.

Usage with RequireJS

I'm lazy so I haven't made it AMD-compatible I'm afraid (feel free to submit a PR!), so you should use a shim config:

  shim: {
    'modernizr-on': {
      deps: ['Modernizr'],
      exports: 'Modernizr'

Although as Modernizr 2.x isn't AMD-compatible either, that'll probably need a shim config too.


If you've got any thoughts about the Modernizr.on() API, please tell us over at the Modernizr issues board, or tweet @Modernizr or @StuCoxMedia.