Cross-domain events in Javascript

As I was telling you in my previous post, we’re currently rewriting most of our Javascript code at PunchTab. As it’s now stable and released for some of our publishers (including this blog), I’m gonna take time to explain the awesome features we’ve built.

The context

As I was saying earlier, we’re dealing with the same technical challenges as Facebook or Google. Our publishers install our Javascript code snippet on their websites, as you would do for a Like button, and it loads our code which then opens some iframes. For many reasons (like connecting the user through Facebook), we have to communicate between the publisher site and our iframes. To achieve this, we’ve been using easyXdm, a cross-domain messaging library. But even so, every time we’ve needed to add a feature using cross-domain communication, it has been painful. That’s why we’ve decided that cross-domain should be one of the core features of our new framework when we started to talk about it.


Our basic usage of easyXdm was the following:

var socket = new easyXDM.Socket({
    remote: 'iframe_you_want_to_open.html',
    onReady: function(){
        socket.postMessage('first message to the iframe');
    onMessage: function(message, origin){
On the other side (in the iframe), the loaded page had quite the same code except the remote.
After several months, we’ve faced some difficulties. First, everybody was defining their own format for the messages, making it harder to understand and to treat. Then we had some sockets created multiple times in different places for the same iframe, with different treatments making hard to track where a message was coming from and who was receiving it. We didn’t want to deal with this communication layer anymore, which was not part of our business logic. So one day, someone said: « what if we could use events everywhere? »


Before coming back to cross-domain, I will have to explain our event layer. If you’ve used jQuery, you probably know how to use events like $(‘.button’).click(callback);. Since the beginning, we’ve wanted our new framework to be event driven, it makes it far easier to write independent modules which can interact with each other by triggering and binding on events. So the first piece was to be able to do this:

PT.event.trigger('myEvent', message) // send an event of type 'myEvent' with a message object

PT.event.bind('myEvent', function(message){ alert(message) }); // when an event of type 'myEvent' is triggered, execute the callback

That part was easily done – it’s a few lines of code. As we were dealing with events, we also decided to solve the most annoying problem of events: their volatility.

Indeed, in the previous example, if I execute the code in this order, nothing will happen because we bind the callback after the event has been triggered so we miss it. In some cases, it doesn’t matter. Like for a click, if the user clicks before we are ready to bind on a click event, it doesn’t matter if we miss it, since the user may click again. But if you take a look back at my previous post, imagine we replace our Facebook init code with this (what we’ve actually done):

var previous_fbAsyncInit = window.fbAsyncInit;
if ((window.fbAsyncInit && window.fbAsyncInit.hasRun) ||
    (!window.fbAsyncInit &&
    window.FB !== undefined &&
    FB.Event !== undefined)) {
} else {
    window.fbAsyncInit = function () {
        if (previous_fbAsyncInit) {

It becomes better since we can just bind on the facebook.ready event for all our features relying on Facebook. But it means they all have to be bound to this event before this piece of code is executed to be safe. What we really want is to execute our callback when facebook.ready happens or execute it directly if facebook.ready has happened in the past.

That’s why we introduced PT.event.persistent(‘facebook.ready’). This triggers and stores the event. Then on a PT.event.bind(), if the event has already happened, it executes the callback directly. We’re typically using it for some events like dom.ready, user.connected, twitter.ready, google.ready, … No need to pay attention in which order you bind on one-time events now.

Propagating events

Ok, now we have an awesome event system. What if we could send all events through easyXdm to the other iframes? That’s what we’ve build by default in the PT.event.trigger function. Whenever we trigger an event, we loop on every iframe we’ve opened through easyXdm and we send them the event serialized in JSON. Then you can trigger and bind on events anywhere and consider all the iframes as the whole and single place. Even if you open an iframe later, we send it the history of persistent events which have already been triggered to make it work the same way.

So now, we just open sockets for cross domain communication and that’s it. All the work is then done with events which makes our life infinitely easier.

To sum up, have a look at this example which sends a message to an iframe through an event:

The code on this side (for explaination about ptReady, see previous post):

<input type="text" id="example" value="Example" />
<input type="submit" id="send" value="Send" />
<div id="iframe-container"></div>
<script type="text/javascript">
window.ptReady = window.ptReady || [];
    document.getElementById('send').onclick = function(){
        return false;
    PT.xdm.socket('iframe', {
        remote: '',
        container: 'iframe-container',

The code in the iframe:

<script type="text/javascript">
        function(message){ document.body.innerHTML = message}

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