Polls for element creation and then loads a new script. Could be used for just about anything including checking for external file loading. Note: there is also MutationObserver but couldn't find anything reliable. https://developer.mozilla.org/en-US/docs/Web/API/MutationObserver

poll_element_loaded.js

var poll;
var timeout = 100; // gives up after 10 seconds

poll = function () {
  setTimeout(function () {
    timeout--;

    if ( $('#myid').length === 1 ) {
      var script= document.createElement('script');
      script.type = 'text/javascript';
      script.src = 'http://mysite.com/js/myscript.js';
      script.async = true;
      document.body.appendChild(script);
    } else if ( timeout > 0 ) {
      poll();
    } else {
      // element failed to load
    }
  }, 100);
};

poll();