matchMedia.js

/*
* MediaQueryListener proof of concept using CSS transitions by Paul Hayes
* November 5th 2011
*
* Based on the excellent matchMedia polyfill
* https://github.com/paulirish/matchMedia.js
*
* matchMedia() polyfill - test whether a CSS media type or media query applies
* authors: Scott Jehl, Paul Irish, Nicholas Zakas
* Copyright (c) 2011 Scott, Paul and Nicholas.
* Dual MIT/BSD license
*/

mql = (function(doc, undefined) {

    var docElem = doc.documentElement,
        refNode = docElem.firstElementChild || docElem.firstChild,
        idCounter = 0;
        if(!doc.getElementById('mq-style')) {
          style = doc.createElement('style');
          style.id = 'mq-style';
          style.textContent = '.mq { -webkit-transition: width 0.001ms; -moz-transition: width 0.001ms; -o-transition: width 0.001ms; -ms-transition: width 0.001ms; width: 0; position: absolute; top: -100em; }\n';
          docElem.insertBefore(style, refNode);          
        }

     var transitionEnds = Array('transitionend','webkitTransitionEnd','oTransitionEnd','msTransitionEnd');
	
     for(var i in transitionEnds) { 
	if ('on'+ transitionEnds[i].toLowerCase() in window)  transitionEnd = transitionEnds[i];
     }	

    return function(q, cb) {

        var id = 'mql-' + idCounter++,
            callback = function() {
                // perform test and send results to callback
                cb({
                    matches: (div.offsetWidth == 42),
                    media: q
                });
            },
            div = doc.createElement('div');

        div.className = 'mq'; // mq class in CSS declares width: 0 and transition on width of duration 0.001ms
        div.id = id;
        style.textContent += '@media ' + q + ' { #' + div.id + ' { width: 42px; } }\n';        

        // add transition event listener
        div.addEventListener(transitionEnd, callback, false); 

        docElem.insertBefore(div, refNode);

        // original polyfill removes element, we need to keep element for transitions to continue and events to fire.
        
        return {
            matches: div.offsetWidth == 42,
            media: q
        };
    };

})(document);