Wait for element to be loaded using MutationObserver

waitForElementLoad.js

function waitForElementLoad(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,
    });
  });
}