How to communicate between an iframe and the parent page

const iframe = document.getElementById('iframe');
iframe.contentWindow.postMessage('some message', '*');

window.onmessage = function(e) {
  if (e.data === 'some message') {
    alert('It works!');
  }
};

window.onmessage = function(e) {
  if (e.data === 'from iframe') {
    alert('It works!');
  }
};

window.top.postMessage('from iframe', '*')