Dynamically require scripts in the browser, with async callback for when all dependencies are loaded

async_require.js

/* Dynamically require scripts in the browser, with async callback for when all dependencies are loaded

Example:

    function pageReady() {
      // Init your code here
      alert('All dependencies loaded!');
    }
    _require(['/json2.js', 'http://server.com/foo.js'], pageReady);

The example above insert 2 script tags in the HEAD for /json2.js and http://server.com/foo.js, asynchronously (doesn't block page rendering while the scripts load).
The pageReady function will run once all scripts have finished loading so it's safe to start your application.
----------------------------------------------------------------------------------*/

var _require = (function () {
  
  var handleScriptLoaded;
  if (document.addEventListener) {
    handleScriptLoaded = function (elem, callback) {
      elem.addEventListener('load', callback, false)
    }
  } else {
    handleScriptLoaded = function(elem, callback) {
      elem.attachEvent('onreadystatechange', function () {
        if(elem.readyState == 'loaded' || elem.readyState == 'complete') callback()
      })
    }
  }
  
  return function (deps, callback) {
    var dep_count = 0,
    dep_length = deps.length;

    function checkReady (callback) {
      dep_count++;
      if ( dep_length == dep_count ) {
        // Opera needs the timeout for page initialization weirdness
        setTimeout(callback, 0);
      }
    }

    function addScript (src, callback) {
      callback = callback || function(){}
      var head = document.getElementsByTagName('head')[0];
      var script = document.createElement('script');
      script.setAttribute('src', src);
      script.setAttribute("type","text/javascript");
      script.setAttribute('async', true);

      handleScriptLoaded(script, function () {
        checkReady(callback);
      });

      head.appendChild(script);
    }   

    for(var i = 0; i < dep_length; i++) {
      addScript(deps[i], callback);
    }
  }
})();