Implement abort

[lukeapage]

As per whatwg/fetch#523

I guess it should wait till the above is merged.

[jimmywarting]

just a reference: https://github.com/mo/abortcontroller-polyfill/blob/master/src/abortcontroller.js#L8-L47
this will patch fetch and return a rejection instead, it dosen't abort the fetch

[coldhamix]

@lukeapage @mislav whatwg/fetch#523 just got merged

[lukeapage]

Yep, I've been following. Good news.

[mislav]

I quite dig the polyfill that @jimmywarting shared. Does anyone think that this project should include that kind of polyfill? Should we try to patch native window.fetch if it doesn't support aborting?

[jimmywarting]

You should try calling xhr.abort to not wast bandwidth

[jamesplease]

I agree with @mislav 's and @jimmywarting 's suggestions over in #572 that keeping the AbortController and AbortSignal polyfills separate from this library seems preferable.

Should we try to patch native window.fetch if it doesn't support aborting?

I think so. You may already be thinking this, but it would probably be a breaking change to the API, as this library would start polyfilling many more apps than it currently does. This might warrant a major version bump if this lib follows semver.

You should try calling xhr.abort to not wast bandwidth

It would be pretty cool to see this library update to accept a (duck-typed) AbortSignal and call xhr.abort whenever the abort event is dispatched.

A quick glance at the code suggests to me that this should be straightforward to implement. Perhaps the hardest part will be updating the feature detection code.

I wrote a quick snippet that seems like a step in the right direction. I tested it in Safari, Chrome, and Firefox, and got the expected results (no, no, yes, respectively).

What do folks think? Should I code up a PR that implements something along these lines? PR'd at #592