Wait for an element to be inserted to the DOM. It tries a query selector on all mutation events.

waitForElementToExist.js

// This assumes that the selector will eventually appear - it'll hang indefinitely otherwise.
// You may need a slight delay after this returns, for the element to have its event handlers attached.

async function waitForElementToExist (selector) {
  return new Promise(resolve => {
    const observer = new MutationObserver(() => {
      const element = document.querySelector(selector)
      if (element) {
        observer.disconnect()
        return resolve(element)
      }
    })

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