Using postmessage to communicate with iframe - where is the error?

Question

I have a Greasemonkey script that should send a postmessage to an embedded iframe where the same script starts a function. My attention is sending an easy message to trigger a function inside the iframe. The website and the iframe are not on the same domain. My js skills are quite bad and I can't find the issue.

Thx for reading

// ==UserScript==
// @name        Test
// @namespace   
// @include     domainA
// @include     domainB
// @version     1
// @grant       none
// ==/UserScript==

if ("domainA" === location.hostname)
{
  if (window === top) // prevents the script from running twice on domain A
  {
    window.setTimeout(delay, 15000);
    function delay()
    {
      console.log("Delay");
      document.getElementsByTagName("Iframe")[0].contentWindow.postMessage('message', 'domainB');  //The issue is probably here
    }
  }  
}
else // domain B 
{ 
 window.onmessage = function()  // or here
 {  
  console.log("Done"); // Didnt start
 }
}

edit: Iam using Firefox

Answer 1

I don't believe you can do this within one script. If you create two scripts it will work. Make sure each script only includes the one domain.

First script that gets the iframe and posts the message to it:

// ==UserScript==
// @name        Test
// @namespace   
// @include     domainA
// @version     1
// @grant       none
// ==/UserScript==

if (window === top) // prevents the script from running twice on domain A
{
  window.setTimeout(delay, 15000);
  function delay()
  {
    console.log("Delay");
    document.getElementsByTagName("Iframe")[0].contentWindow.postMessage('message', 'domainB');  //The issue is probably here
  }
}

Second script that matches the domain of the iframe and attaches the event handler:

// ==UserScript==
// @name        Test IFrame
// @namespace   
// @include     domainB
// @version     1
// @grant       none
// ==/UserScript==

if (window.addEventListener) 
{
    window.addEventListener("message", function (event) 
    {  
        console.log("Done");
    }
}
else  // IE8 or earlier
{
    window.attachEvent("onmessage", function (event)
    {
        console.log("Done");
    }
}