waitFor MutationObserver function

waitFor.js

  function waitFor(selector, cb) {
    // set up the mutation observer
    const observer = new MutationObserver(function (mutations, me) {
      // `mutations` is an array of mutations that occurred
      // `me` is the MutationObserver instance
      var element = document.querySelector(selector)
      if (element) {
        cb(element)
        me.disconnect() // stop observing
        return
      }
    });

    // start observing
    observer.observe(document, {
      childList: true,
      subtree: true
    })
  }