Difference between custom event and postMessage

Question

In order to send a message to another document (let's say an iframe), you can use both postMessage and createEvent functions. Assume this:

var event = document.createEvent('CustomEvent');
event.initCustomEvent("message", true, true, 'Hello world');
iframe.dispatchEvent(event);

My question is, if both approaches work, what is the difference between using postMessage and customEvent?

Amadan · Accepted Answer · 2022-06-09T14:47:55.087

25

It's the difference between leaving your neighbour a message asking them to turn down their TV, and trying to break into their apartment to turn down the TV yourself.

You can't dispatch an event into a frame that you are not allowed to access by Same Origin Policy or Access-Control-Allow-Origin, since some of the messages might mess with how that page works. But messages are intended for communication between different pages - if they don't want to listen to the message, they don't have to.

Another difference is that messages must be serialisable, events do not have to be.

edited Jun 09 '22 at 14:47

answered May 12 '15 at 06:49

Amadan

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And what if we are in the same domain name? – Afshin Mehrabani May 12 '15 at 06:50
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Then the results should be similar. Your apartment, your TV. Doesn't matter if you tell your kid to turn it down or do it yourself. – Amadan May 12 '15 at 06:51
Awesome. What I was looking for. – Afshin Mehrabani May 12 '15 at 06:52
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Best example ever – Manel Clos Sep 21 '18 at 08:52
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Well more like throwing a stone at their TV hoping it would stop. It might or might not stop, but if the host is gracious enough - they would listen to the "request" and turn it down themselves. – bendtherules Feb 12 '20 at 11:58
@bendtherules I agree. "... turning down the TV yourself." comes off as if someone can manually modify another page. – Luminous Jun 09 '22 at 13:51
@Luminous Edited. You're still breaking in, but maybe the security is good enough to stop you. – Amadan Jun 09 '22 at 14:48

Difference between custom event and postMessage

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