dynamic script with callback

dynamicScript,js

function dynamicAsset(script_name, cb) {
  var body = document.body,
    script = document.createElement('script'),
    done = false;

  script.type = 'text/javascript';
  script.src = lang.templateUrl + '/dist/scripts/' + script_name;
  script.onload = script.onreadystatechange = function(){

  if ( !done && (!this.readyState ||
    this.readyState === "loaded" || this.readyState === "complete") ) {
      done = true;

      if (!!(cb && cb.constructor && cb.call && cb.apply)) {
        cb();
      }
    }
  };
  body.appendChild(script);
}