Вставка дочернего файла js в родительский js файл.

script.js

//jqery method:
$.getScript('js/_child-script.js', function() {/*dependet code*/});

//js method:
var _childScript = document.createElement('script');
_childScript.src = '/js/_child-script.js';
document.head.appendChild(_cart);

_childScript.onload = function() {
//The library is loaded but not yet initialized, 
//its code has not yet been executed, 
//so we plan to launch our code in the next tick (setTimeout())
  setTimeout(function() {
    /*dependet code*/
  }, 0);
};