explainer/spec: mapfn may be async

See #4.

Author: js-choi

README.md

@@ -59,10 +59,19 @@ await Array.fromAsync(f());
 
 [`Array.from`]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/from
 
-`mapfn` is an optional function to call on every item value,
-and `thisArg` is an optional value to (or `undefined` by default).
-
-Also similarly to `Array.from`, `Array.fromAsync` is a **generic factory method**.
+`mapfn` is an optional function to call on every item value.
+(Unlike `Array.from`, `mapfn` may be an **async function**.
+Whenever `mapfn` returns a promise, that promise will be awaited,
+and the value it resolves to is what is added
+to the final returned promise’s array.
+If `mapfn`’s promise rejects,
+then the final returned promise
+will also reject with that error.)
+
+`thisArg` is an optional value with which to call `mapfn`
+(or `undefined` by default).
+
+Like `Array.from`, `Array.fromAsync` is a **generic factory method**.
 It does not require that its `this` value be the `Array` constructor,
 and it can be transferred to or inherited by any other constructors
 that may be called with a single numeric argument.

spec.html

@@ -69,7 +69,9 @@ <h1><ins>Array.fromAsync ( _asyncItems_ [ , _mapfn_ [ , _thisArg_ ] ] )</ins></h
                 1. Let _nextValue_ be ? IteratorValue(_next_).
                 1. If _mapping_ is *true*, then
                   1. Let _mappedValue_ be Call(_mapfn_, _thisArg_, &laquo; _nextValue_, 𝔽(_k_) &raquo;).
-                  1. If _mappedValue_ is an abrupt completion, return ? AsyncIteratorClose(_iteratorRecord_, _mappedValue_).
+                  1. IfAbruptCloseAsyncIterator(_iteratorRecord_, _mappedValue_).
+                  1. Let _mappedValue_ to Await(_mappedValue_).
+                  1. IfAbruptCloseAsyncIterator(_iteratorRecord_, _mappedValue_).
                   1. Set _mappedValue_ to _mappedValue_.[[Value]].
                 1. Else, let _mappedValue_ be _nextValue_.
                 1. Let _defineStatus_ be CreateDataPropertyOrThrow(_A_, _Pk_, _mappedValue_).