Re: [heycam/webidl] Import the Promises Guide. (#772)

domenic commented on this pull request.



>          1.  Let |R| be the first argument to |onRejected|.
         1.  Let |reason| be the result of [=converted to an IDL value|converting=]
             |R| to an IDL value of type {{any}}.
         1.  If there are no steps that are required to be run if the promise was rejected, then
             return <emu-val>undefined</emu-val>.
         1.  Otherwise, return the result of performing any steps that were required to be run if the promise was rejected,
             with |reason| as the rejection reason.
-    1.  Return [=!=] [$PerformPromiseThen$](|promise|, |onFulfilled|, |onRejected|).
+    1.  Let |onRejected| be [=!=] [$CreateBuiltinFunction$](|onRejectedSteps|, « »):
+    1.  Let |constructor| be |promise|.\[[Promise]].\[[Realm]].\[[Intrinsics]].[[{{%Promise%}}]].
+    1.  Let |newCapability| be ? [$NewPromiseCapability$](|constructor|).
+    1.  Return [=!=] [$PerformPromiseThen$](|promise|.\[[Promise]], |onFulfilled|, |onRejected|, |newCapability|).

Well, current "upon fulfillment" etc. usages don't have a return value... that's where you'd use "transforming". But I see you seem to be allowing this to stand in for both, hmm.

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