Implement Array.from()

src/harmony-array.js

Index: src/harmony-array.js
diff --git a/src/harmony-array.js b/src/harmony-array.js
index 88b878f0a76ef594f64f7905d61459c05749724b..6dc8337dccf419f53c41826de0ccb44534bbafa6 100644
--- a/src/harmony-array.js
+++ b/src/harmony-array.js
@@ -123,6 +123,66 @@ function ArrayFill(value /* [, start [, end ] ] */) {  // length == 1
   return array;
 }
 
+// ES6, draft 07-18-14, section 22.1.2.1
+function ArrayFrom(arrayLike, mapfn, receiver) {
+  var C = this;
+  var items = ToObject(arrayLike);
+  var mapping;
+
+  if (IS_UNDEFINED(mapfn)) {
+    mapping = false;
+  } else {
+    mapping = true;
+    if (!IS_SPEC_FUNCTION(mapfn)) {
+      throw MakeTypeError('called_non_callable', [ mapfn ]);
+    }
+    if (IS_NULL_OR_UNDEFINED(receiver)) {
+      receiver = %GetDefaultReceiver(mapfn) || receiver;
+    } else if (!IS_SPEC_OBJECT(receiver) && %IsSloppyModeFunction(mapfn)) {
+      receiver = ToObject(receiver);
+    }
+  }
+
+  var usingIterator = ToIterable(items);
+  var k;
+  var A;
+  var mappedValue;
+  var nextValue;
+
+  if (!IS_UNDEFINED(usingIterator)) {
+    // Step 8.
+    A =  IS_SPEC_FUNCTION(C) ? new C() : [];
+
+    var iterator = GetIterator(items, usingIterator);
+    var next;
+    k = 0;
+
+    while (!(next = iterator.next()).done) {
+      if (!IS_SPEC_OBJECT(next)) {
+        throw MakeTypeError('iterator_result_not_an_object', [next]);
+      }
+      nextValue = next.value;
+      mappedValue = mapping ? %_CallFunction(receiver, nextValue, k, mapfn) : nextValue;
+      %AddElement(A, k++, mappedValue, NONE);
+    }
+    A.length = k;
+    return A;
+  } else {
+    // Steps 9 .. 20
+    var len = TO_LENGTH(items.length);
+    A = IS_SPEC_FUNCTION(C) ? new C() : [];
+
+    for (k = 0; k < len; ++k) {
+      nextValue = items[k];
+      mappedValue = mapping ? %_CallFunction(receiver, nextValue, k, mapfn) : nextValue;
+      %AddElement(A, k, mappedValue, NONE);
+    }
+
+    A.length = k;
+    return A;
+  }
+}
+
 // ES6, draft 05-22-14, section 22.1.2.3
 function ArrayOf() {
   var length = %_ArgumentsLength();
@@ -141,8 +201,11 @@ function ArrayOf() {
 function HarmonyArrayExtendArrayPrototype() {
   %CheckIsBootstrapping();
 
+  %FunctionSetLength(ArrayFrom, 1);
+
   // Set up non-enumerable functions on the Array object.
   InstallFunctions($Array, DONT_ENUM, $Array(
+    "from", ArrayFrom,
     "of", ArrayOf
   ));