Cross-browser way to check if document has loaded (vanilla JS)From http://www.jstips.co/en/detect-document-ready-in-pure-js/

document-ready.js

// The cross-browser way to check if the document has loaded in pure JavaScript is using readyState.
if (document.readyState === 'complete') {
    // The page is fully loaded
}

// You can detect when the document it's ready...
let stateCheck = setInterval(() => {
    if (document.readyState === 'complete') {
    clearInterval(stateCheck);
     // document ready
  }
}, 100);

// or with onreadystatechange...
document.onreadystatechange = () => {
  if (document.readyState === 'complete') {
   // document ready
  }
};
// Use document.readyState === 'interactive' to detect when the DOM is ready.