Check out example codes for "html5 mssenger". It will help you in understanding the concepts better.

Code Example 1

var iframe = document.querySelector('iframe');
var button = document.querySelector('button');

var clickHandler = function() {
   iframe.contentWindow.postMessage('The message to send.',
      'https://www.tutorialspoint.com);
}
button.addEventListener('click',clickHandler,false);

Code Example 2

var messageEventHandler = function(event){
   
   // check that the origin is one we want.
   if(event.origin == 'https://www.tutorialspoint.com') {
      alert(event.data);
   }
}
window.addEventListener('message', messageEventHandler,false);

Code Example 3

var loadHandler = function() {
   var mc, portMessageHandler;
   mc = new MessageChannel();
   window.parent.postMessage('documentAHasLoaded','http://foo.example',[mc.port2]);
   
   portMessageHandler = function(portMsgEvent) {
      alert( portMsgEvent.data );
   }
   
   mc.port1.addEventListener('message', portMessageHandler, false);
   mc.port1.start();
}
window.addEventListener('DOMContentLoaded', loadHandler, false);

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