JavaScript: window.postMessage API

gistfile1.js

// From window or frame on domain 1, send a message to the iframe which hosts another domain
var iframeWindow = document.getElementById("iframe").contentWindow;
iframeWindow.postMessage("Hello from the first window!");

// From inside the iframe on different host, receive message
window.addEventListener("message", function(event) {
  // Make sure we trust the sending domain
  if(event.origin == "http://davidwalsh.name") {
    // Log out the message
    console.log(event.data);

    // Send a message back
    event.source.postMessage("Hello back!");
  }
]);