element-exists.js

// Wait for an element to be ready uses Mutation Observer and Promises.
function elementReady(selector) {
  return new Promise((resolve, reject) => {
  let el = document.querySelector(selector);
  if (el) {
    resolve(el); 
    return
  }
  new MutationObserver((mutationRecords, observer) => {
    // Query for elements matching the specified selector
    Array.from(document.querySelectorAll(selector)).forEach((element) => {
      resolve(element);
      //Once we have resolved we don't need the observer anymore.
      observer.disconnect();
    });
  })
    .observe(document.documentElement, {
      childList: true,
      subtree: true
    });
});
}