Internet Explorer only takes string in HTML5 postMessage

Today I learned the hard way that our beloved Internet Explorer doesn’t handle HTML5 cross-domain messaging functionality (AKA postMessage) correctly.

I have done a reasonable amount of research before I invested a significant time into this, and so I knew it doesn’t postMessage across tabs and windows beforehand. This limitation was not a problem for me, because I was going to use this in IFRAME.

The problem that I discovered much too late is that IE only accepts a string as a message (see BSTR in a signature? That’s a Windows speak for passing UTF-16 strings.) This is despite the fact that the draft spec allowed any object and not just strings as early as Novemember 2010 .

And other sites like and various blog posts failed me too, as they failed to mention this limitation.

So I’m posting this here in the hope that this will prevent other people from wasting time.


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