explainer/spec: Sync iterators and array-like objects

Fixes #6. Also addresses #7.

README.md

@@ -46,9 +46,9 @@ Similarly to **[`Array.from`][]**,
 of the `Array` built-in class, with **one required argument**
 and **two optional arguments**: `(items, mapfn, thisArg)`.
 
-But instead of converting an **iterable** to an array,
-it converts an **async iterable** to a **promise**
-that will resolve to an array.
+But instead of converting an **array-like object** or **iterable** to an array,
+it converts an **async iterable** (or array-like object or iterable)
+to a **promise** that will resolve to an array.
 
 ```js
 async function * f () {

spec.html

@@ -43,19 +43,20 @@ <h1><ins>Array.fromAsync ( _asyncItems_ [ , _mapfn_ [ , _thisArg_ ] ] )</ins></h
         <emu-alg>
           1. Let _C_ be the *this* value.
           1. Let _promiseCapability_ be ! NewPromiseCapability(%Promise%).
-          1. If _mapfn_ is *undefined*, let _mapping_ be *false*.
-          1. Else,
-            1. If IsCallable(_mapfn_) is *false*, throw a *TypeError* exception.
-            1. Let _mapping_ be *true*.
-          1. Let _usingAsyncIterator_ be ? GetMethod(_asyncItems_, @@asyncIterator).
-          1. If _usingAsyncIterator_ is *undefined*, throw a *TypeError* exception.
-          1. Else,
-            1. If IsConstructor(_C_) is *true*, then
-              1. Let _A_ be ? Construct(_C_).
+          1. Let _fromAsyncClosure_ be a new Abstract Closure with no parameters that captures _C_ and _mapfn_ and performs the following steps when called:
+            1. If _mapfn_ is *undefined*, let _mapping_ be *false*.
+            1. Else,
+              1. If IsCallable(_mapfn_) is *false*, throw a *TypeError* exception.
+              1. Let _mapping_ be *true*.
+            1. Let _usingAsyncOrSyncIterator_ be ? GetMethod(_asyncItems_, @@asyncIterator).
+            1. If _usingAsyncOrSyncIterator_ is *undefined*, let _usingAsyncOrSyncIterator_ be ? GetMethod(_asyncItems_, @@iterator).
+            1. If _usingAsyncOrSyncIterator_ is *undefined*, throw a *TypeError* exception.
             1. Else,
-              1. Let _A_ be ! ArrayCreate(0).
-            1. Let _iteratorRecord_ be ? GetIterator(_asyncItems_, ~async~, _usingAsyncIterator_).
-            1. Let _asyncSpreadClosure_ be a new Abstract Closure with no parameters that captures _A_ and _mapfn_ and performs the following steps when called:
+              1. If IsConstructor(_C_) is *true*, then
+                1. Let _A_ be ? Construct(_C_).
+              1. Else,
+                1. Let _A_ be ! ArrayCreate(0).
+              1. Let _iteratorRecord_ be ? GetIterator(_asyncItems_, ~async~, _usingAsyncOrSyncIterator_).
               1. Let _k_ be 0.
               1. Repeat,
                 1. If _k_ &ge; 2<sup>53</sup> - 1, then
@@ -77,8 +78,26 @@ <h1><ins>Array.fromAsync ( _asyncItems_ [ , _mapfn_ [ , _thisArg_ ] ] )</ins></h
                 1. Let _defineStatus_ be CreateDataPropertyOrThrow(_A_, _Pk_, _mappedValue_).
                 1. If _defineStatus_ is an abrupt completion, return ? AsyncIteratorClose(_iteratorRecord_, _defineStatus_).
                 1. Set _k_ to _k_ + 1.
-            1. Perform AsyncFunctionStart(promiseCapability, _asyncSpreadClosure_).
-            1. Return Completion { [[Type]]: ~return~, [[Value]]: _promiseCapability_.[[Promise]], [[Target]]: ~empty~ }.
+            1. NOTE: _items_ is not an AsyncIterable or Iterable so assume it is an array-like object.
+            1. Let _arrayLike_ be ! ToObject(_items_).
+            1. Let _len_ be ? LengthOfArrayLike(_arrayLike_).
+            1. If IsConstructor(_C_) is *true*, then
+              1. Let _A_ be ? Construct(_C_, &laquo; 𝔽(_len_) &raquo;).
+            1. Else,
+              1. Let _A_ be ? ArrayCreate(_len_).
+            1. Let _k_ be 0.
+            1. Repeat, while _k_ &lt; _len_,
+              1. Let _Pk_ be ! ToString(𝔽(_k_)).
+              1. Let _kValue_ be ? Get(_arrayLike_, _Pk_).
+              1. If _mapping_ is *true*, then
+                1. Let _mappedValue_ be ? Call(_mapfn_, _thisArg_, &laquo; _kValue_, 𝔽(_k_) &raquo;).
+              1. Else, let _mappedValue_ be _kValue_.
+              1. Perform ? CreateDataPropertyOrThrow(_A_, _Pk_, _mappedValue_).
+              1. Set _k_ to _k_ + 1.
+            1. Perform ? Set(_A_, *"length"*, 𝔽(_len_), *true*).
+            1. Return _A_.
+          1. Perform AsyncFunctionStart(promiseCapability, _fromAsyncClosure_).
+          1. Return Completion { [[Type]]: ~return~, [[Value]]: _promiseCapability_.[[Promise]], [[Target]]: ~empty~ }.
         </emu-alg>
         <emu-note>
           <p>The `fromAsync` function is an intentionally generic factory method; it does not require that its *this* value be the Array constructor. Therefore it can be transferred to or inherited by any other constructors that may be called with a single numeric argument.</p>