Append to "onload" function in JS

onLoadAdder.js

//source: https://ehikioya.com/working-with-javascript-onload-functions/
//Add function to onload without overriding previously defined onloads
AddOnLoadEvent(yourFunction);

function AddOnLoadEvent(functionX) {
  var oldonloadevent = window.onload;
  if (typeof window.onload != 'function') {
    window.onload = functionX;
  }
  else {
    window.onload = function () {
      if (oldonloadevent) {
        oldonloadevent();
      }
      functionX();
    }
  }
}