If you are adding your script file dynamically, try adding async attribute. This way your script will load after your dom loading is finished and any reference to your dom elements will not throw 'undefined' error.

<script>
     
  function addScripts() {
    var headObj = document.getElementsByTagName("body")[0];
    var scriptFile = document.createElement("script");
    scriptFile.type = 'text/javascript';
    scriptFile.src = "webwb/yourScriptFile.js";
    scriptFile.async = true; // Load asynchronously
    headObj.appendChild(scriptFile);
  }     
   window.addEventListener("load", addScripts);
  
</script>

Hi Andrey,

You can make use of this ootb onload function as well. You can use the function like below. 

pega.u.d.attachOnload(alert("hi"));