iframe page — the document living inside the frame.

Its JavaScript, plainly: a hello() function that alerts it, plus one message listener (the postMessage side).

function hello(str) { window.alert(str); }
window.addEventListener('message', function (e) {
  if (e.data && e.data.message) { window.alert(e.data.message); }
});

Right-click → View source works on frames exactly like pages.