zero dependency $.getScript for loading large external javascripts

bootstrap_getScript.js

// zero dependency $.getScript for loading external javascript (like jQuery.js
// or other big libraries) in a way that does not block the normal page load
// (http://www.yuiblog.com/blog/2008/07/22/non-blocking-scripts/).
//
// heavily influenced by/copied from jQuery's ajax functions (http://jquery.com)

window.$ = {
  getScript: function(script_src, callback) {
    var done = false;
    var head = document.getElementsByTagName("head")[0] || document.documentElement;
    var script = document.createElement("script");
    script.src = script_src;
    script.onload = script.onreadystatechange = function() {
      if ( !done && (!this.readyState ||
          this.readyState === "loaded" || this.readyState === "complete") ) {
        if(callback) callback();

        // Handle memory leak in IE
        script.onload = script.onreadystatechange = null;
        if ( head && script.parentNode ) {
          head.removeChild( script );
        }
        
        done = true;
      }
    };
    head.insertBefore( script, head.firstChild );
  }
};