Load a script from a URL, with before/after callbacks & queueing

load-script.js

// Load a script from a URL, with callbacks
// If a 'before' callback is specified, script will load synchronously
{
  window.scriptsLoaded = {};
  window.loadScriptOnce = function(src, callback, beforeRun) {
    if(typeof window.scriptsLoaded[src] === 'undefined') {
      window.scriptsLoaded[src] = [];
      var tag = document.createElement('script');
      tag.src = src;

      if(beforeRun) {
        tag.async = false;
        beforeRun();
      }

      if(typeof callback === 'function') {
        window.scriptsLoaded[src].push(callback);
        if (tag.readyState) { // IE, incl. IE9
          tag.onreadystatechange = (function() {
            if (tag.readyState == "loaded" || tag.readyState == "complete") {
              tag.onreadystatechange = null;
              for(var i = 0; i < window.scriptsLoaded[this].length; i++) {
                window.scriptsLoaded[this][i]();
              }
              window.scriptsLoaded[this] = true;
            }
          }).bind(src);
        } else {
          tag.onload = (function() { // Other browsers
            for(var i = 0; i < window.scriptsLoaded[this].length; i++) {
              window.scriptsLoaded[this][i]();
            }
            window.scriptsLoaded[this] = true;
          }).bind(src);
        }
      }

      var firstScriptTag = document.getElementsByTagName('script')[0];
      firstScriptTag.parentNode.insertBefore(tag, firstScriptTag);
      return true;
    } else if(typeof window.scriptsLoaded[src] === 'object' && typeof callback === 'function') {
      window.scriptsLoaded[src].push(callback);
    } else {
      if(typeof callback === 'function') {
        callback();
      }
      return false;
    }
  }
}