Question

window.onmessage = ...
window.postMessage('1', '*');
window.postMessage('2', '*');

Does postMessage (http://www.whatwg.org/specs/web-apps/current-work/multipage/webappapis.html#queue-a-task) guarantee the order of events?

Answer 1

I don’t know, although the wording of the spec seems to suggest it doesn’t make such guarantees (which is surprising). It’s clear that once the MessageEvent is added to the task queue on the receiving end, then it’s order is maintained, although the MessageEvent creation and dispatch are asynchronous to the original postMessage call, so theoretically it appears that you could have the following situation:

main thread:

window.postMessage('1', '*'); --> thread spawned to create MessageEvent
window.postMessage('2', '*'); --> new thread spawned for another MessageEvent

If the thread management system allowed the second postMessage to execute before the first thread managed to dispatch the MessageEvent, and for whatever unlucky reason allowed that newer thread to execute (a diluted priority inversion), again before the first managed to dispatch, then you would indeed receive those messages in the reverse order.

Although there might be some other place in the spec that provides more context for these asynchronous executions and rules out this case – I couldn’t find it.