A quick snippet using Javascript's requestAnimationFrame to determine when the document is ready or a library is loaded.

doOnLoad.js

const ready = () => {
  
  // Example of checking if library is available
  if ('jQuery' in window) {
    return;
  }
  
  // Example of checking if document is loaded
  if (document.body) {
    // Do code here...
    return;
  }
  
  // Call again if not found
  window.requestAnimationFrame(ready);
};

window.requestAnimationFrame(ready);