JavaScript Loader

loader.js

function jsLoader() {
  "use strict";

  var scripts = [].slice.call(arguments);
  var reserves = [];
  var thisScript = document.scripts[0] || document.querySelector("script");
  var head = thisScript.parentNode;

  if ("async" in thisScript)
    parallel();
  else if (thisScript.readyState)
    parallelForIE();
  else
    sequential();

  function parallel() {
    scripts.forEach(function (script) {
      var elem = document.createElement("script");
      elem.async = false;
      elem.src = script;
      head.insertBefore(elem, thisScript);
    });
  }
  function parallelForIE() {
    scripts.forEach(function (script) {
      var elem = document.createElement("script");
      reserves.push(elem);
      elem.onreadystatechange = stateChange;
      elem.src = script;
    });
  }
  function sequential() {
    var script = scripts.shift();
    var elem;
    if (script) {
      elem = document.createElement("script");
      elem.src = script;
      elem.onload = sequential;
      head.insertBefore(elem, thisScript);
    }
  }
  function stateChange() {
    var elem;
    while (reserves[0] && reserves[0].readyState === "loaded") {
      elem = reserves.shift();
      elem.onreadystatechange = null;
      head.insertBefore(elem, thisScript);
    }
  }
}