// child
document.addEventListener('DOMContentLoaded', () => {
window.parent.notifyAboutReady();
});
window.updateValue = (value) => {
document.getElementById('counter').innerText = value;
}
// parent
window.notifyAboutReady = () => {
document.getElementById('is-ready-container').innerText = 'Container is ready'
}
document.addEventListener('DOMContentLoaded', () => {
const frame = window.frames['container'];
let i = 0;
setInterval(() => frame.updateValue(i++), 1_000);
});