iframe scroll to bottom detection (cross browser, including mobile safari)

client.js

window.addEventListener('scroll', handleScroll);
window.addEventListener('message', handleMessage);

handleMessage = (event) => {
  if (event.data === 'scrolledToBottom') handleScrolledToBottom();
}

handleScroll = () => {
  if ((window.innerHeight + window.pageYOffset) >= window.document.body.scrollHeight) {
    this.handleScrolledToBottom();
  }
}

handleScrolledToBottom = () => {
  // do something
}

iframe.js

const handleScroll = (event) => {
  if ((window.innerHeight + window.pageYOffset) >= document.body.scrollHeight) {
    window.parent.postMessage('scrolledToBottom', '*')
  }
}

window.addEventListener('scroll', handleScroll);