simple-loader.js

var fetch = (function () {
  var scriptStatus = {},
      callbackStack = [],
      STATUS_LOADING = 1,
      STATUS_LOADED = 2;
  
  function requestScript(url) {
    if (typeof(scriptStatus[url]) === 'undefined') {
      var scriptTag = document.createElement('script');
      scriptTag.type = 'text/javascript';
      scriptTag.src = url;
      scriptStatus[url] = STATUS_LOADING;
    
      scriptTag.onload = function () {
        markLoaded(url);
      };
    
      document.body.appendChild(scriptTag);
    }
  }
  
  function markLoaded(url) {
    scriptStatus[url] = STATUS_LOADED;
    
    callbackStack.forEach(function (callbackData, i) {
      if (callbackData.scripts.every(function (url) {
        return scriptStatus[url] && scriptStatus[url] === STATUS_LOADED;
      }) {
        callbackData.callback();
        delete callbackStack[i];
      }
    });
  }
  
  return function(scripts, callback) {
    var scriptsToLoad = scripts.filter(function (url) {
      return !scriptStatus[url] || scriptStatus[url] !== STATUS_LOADED;
    });
    
    if (!scriptsToLoad.length)
      return callback();
    }
    
    scriptsToLoad.forEach(function (url) {
      requestScript(url);
    });
    
    callbackStack.push({
      callback: callback,
      scripts: scripts
    });
  }
}());