Cancelable promises proposal

Glen Huang curvedmark at
Tue Aug 4 00:15:42 UTC 2015

This is a good point. I didn't think about passing the same callback more than once.

Ideally, if the same resolve & reject callback combination is passed more than once, the same promise is always returned (I can't think of a use case where promises could stop to work, if promises were specced with this additional rule, but maybe I overlooked). So if we pretend that's the case, it seems ignore() should deregister all callbacks.

Do you have a use case where the same callback will be passed to the same promise more than once? Does deregistering all callbacks at once seem counter-intuitive in that case if you want to show disinterest?

Another way out is to make ignore() cancel then() in a LIFO manner, probably also provide a ignoreAll(), which also gives a hint that ignore() only ignore one callback combination a time. But this just seems ugly.

> On Aug 3, 2015, at 11:49 PM, Boris Zbarsky <bzbarsky at> wrote:
> On 8/2/15 8:43 PM, Glen Huang wrote:
>> You can think it as that each promise keeps a list of its child promises, when the same callback is passed to .ignore() it sets a flag on the corresponding child promise so that when itself resolves/rejects, it won't pass that state to that child promise
> There seems to be a potential footgun here.  Say I call then() twice with the same function.  Now I have two child promises.  If I now call ignore() with that function, which of the child promises is ignored? Both?  Just one of them?  Something else?
> This situation doesn't _quite_ arise with add/removeEventListener, because you can't addEventListener a given listener twice to the same event target.  So a removeEventListener can see at most one matching listener.
> -Boris
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