Re: [whatwg/dom] Abort controller (#437)

jyasskin commented on this pull request.



> + {{DOMException}}.
+ <li>Reject immediately if the {{AbortSignal}}'s [=AbortSignal/aborted flag=] is already set,
+ otherwise:
+ <li>Use the [=AbortSignal/abort algorithms=] mechanism to observe changes to the {{AbortSignal}}
+ object and do so in a manner that does not lead to clashes with other observers.
+</ul>
+
+<div class=example id=aborting-ongoing-activities-spec-example>
+ The steps for a promise-returning method <code>doAmazingness(options)</code> could be as follows:
+
+ <ol>
+  <li>Let |p| be [=a new promise=].
+  <li>If |options|' <code>signal</code> member is present, then:
+   <ol>
+    <li>If |options|' <code>signal</code>'s [=AbortSignal/aborted flag=] is set, throw an
+    "{{AbortError}}" {{DOMException}}.

I think it's not as widely appreciated as it should be and could use a note. Folks tend not to know about the "If op has a return type that is a promise type" part of https://heycam.github.io/webidl/#dfn-create-operation-function.

Or you could be less subtle: wait to create *p* until you need to store it or go *in parallel*, and "return [a promise rejected with](https://www.w3.org/2001/tag/doc/promises-guide#a-promise-rejected-with) ..." here.

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