Goal

Provide developers with a method to abort something initiated with fetch() in a way that is not overly complicated.

Previous discussion

• Add timeout option #20
• Cancelling HTTP fetch w3c/ServiceWorker#592
• Returning a FetchPromise from fetch() w3c/ServiceWorker#625
• Getting errored/closed reader, Auto-releasing reader, Forcible cancel() on a stream streams#297
• Potential security issues when applied to fetch() tc39/proposal-cancelable-promises#4 (security issues)

Viable solutions

We have two contenders. Either fetch() returns an object that is more than a promise going forward or fetch() is passed something, either an object or a callback that gets handed an object.

A promise-subclass

In order to not clash with cancelable promises (if they ever materialize) we should pick a somewhat unique method name for abortion. I think terminate() would fit that bill.

var f = fetch(url)
f.terminate()

Note: the Twitter-sphere seemed somewhat confused about the capabilities of this method. It would most certainly terminate any ongoing stream activity as well. It's not limited to the "lifetime" of the promise.

A controller

The limited discussion on es-discuss https://esdiscuss.org/topic/cancelable-promises seemed to favor a controller. There are two flavors that keep coming back. Upfront construction:

var c = new FetchController
fetch(url, {controller: c})
c.abort()

Revealing constructor pattern:

fetch(url, {controller: c => c.abort()})

Open issues

• What is the effect on the promise? Both forever-pending and explicit rejection have reasonable arguments. We could offer the choice to the developer, but what should be the default?
• What is the effect on the stream? I suspect the Streams Standard is already conclusive on this.
• What syntax of the above two-three solutions do we favor?