window.postMessage

HTML

<a href="#">Gimme a message!</a>

JavaScript

window.addEventListener('message', receiver, false);
function receiver(e) {

  if (e.origin == 'http://fiddle.jshell.net/') {
    if (e.data == 'Hello world') {
      e.source.postMessage('Hello', e.origin);
    } else {
      alert(e.data);
    }
  }
}

document.querySelector( 'a' ).onclick = function() {
    window.postMessage( 'This is a message.', 'jsfiddle.net/' );
};