Implement .fill() on typed arrays

src/harmony-typedarray.js

 // Copyright 2014 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 'use strict';
 
 // This file relies on the fact that the following declaration has been made
 // in runtime.js:
 // var $Array = global.Array;
 
@@ skipping 40 matching lines @@
 
   var stepping = DEBUG_IS_ACTIVE && %DebugCallbackSupportsStepping(f);
   for (var i = 0; i < length; i++) {
     var element = this[i];
     // Prepare break slots for debugger step in.
     if (stepping) %DebugPrepareStepInIfStepping(f);
     var new_receiver = needs_wrapper ? ToObject(receiver) : receiver;
     %_CallFunction(new_receiver, TO_OBJECT_INLINE(element), i, this, f);
   }
 }
+
+
+// ES6 draft 10-14-2014, section 22.1.3.6
+function NAMEFill(value /* [, start [, end ] ] */) {  // length == 1
+  CHECK_OBJECT_COERCIBLE(this, "NAME.prototype.fill");
+
+  var array = ToObject(this);

[wingo, 2014/11/12 09:29:56]

Need a check that the object is indeed a typed array.  See
http://people.mozilla.org/~jorendorff/es6-draft.html#sec-%typedarray%.prototy....
 No check needed that the array is of any particular kind, though.  (Didn't we
go through this on a different patch?  I don't recall.)

[aperez, 2014/11/13 16:34:36]

well, CHECK_OBJECT_COERCIBLE() just checks that the
arguments is not null/undefined, and that is not an
“undetectable object”, so this is not checking for a
typed array, just making sure the argument is something
we can operate on.

In the case of .fill() we do not have to check that the
receiver is a typed array. Or, well, the spec does not say
a thing about such check, and moreover it says at the bottom
of the section:

   “The fill function is intentionally generic; it does
   not require that its this value be an Array object.
   Therefore it can be transferred to other kinds of
   objects for use as a method.”

Therefore, I believe the code is correct as-is.

[wingo, 2014/11/14 14:27:50]

Please see
http://people.mozilla.org/~jorendorff/es6-draft.html#sec-%25typedarray%25.pro...,
which I pasted above.  The typed array fill function is not generic.

+  var length = ToLength(array.length);

[wingo, 2014/11/12 09:29:56]

Need to get the array's "length" internal slot, not the length property.

[aperez, 2014/11/13 16:34:36]

The spec, in step 3 reads:

  3. Let lenVal be Get(O, "length").

…and Get() is specified to retrieve values of properties.
There is no reference to internal slots at all in the
definition of %TypedArray%.prototype.fill(), so I believe
the current code to be correct.

+
+  var i = 0;
+  var end = length;
+
+  if (%_ArgumentsLength() > 1) {
+    i = %_Arguments(1);
+    i = IS_UNDEFINED(i) ? 0 : TO_INTEGER(i);

[wingo, 2014/11/12 09:29:56]

I think that here we need i = TO_INTEGER(i) in all cases; "undefined" isn't
special.

[aperez, 2014/11/13 16:34:36]

Well, according to the spec, the first step of ToInteger()
is to call ToNumber(), which returns NaN for undefined, and
ToInteger() will then return 0 for NaN itself... So, the
bottom line is that there is no no need to check for
undefined because ToInteger(undefined) → 0.

I will be changing this to just: i = TO_INTEGER(i)

+    if (%_ArgumentsLength() > 2) {
+      end = %_Arguments(2);
+      end = IS_UNDEFINED(end) ? length : TO_INTEGER(end);

[wingo, 2014/11/12 09:29:56]

Very strangely, this logic is correct for "end" but not for "start".  Spec WTF?

[aperez, 2014/11/13 16:34:36]

For the case of end==undefined we want to take the length
of the array, and if ToInteger() would be used directly,
then we would get 0 for undefined (see previous comment).

It seems the spec is okay after all...

+    }
+  }
+
+  if (i < 0) {
+    i += length;
+    if (i < 0) i = 0;
+  } else {
+    if (i > length) i = length;
+  }
+
+  if (end < 0) {
+    end += length;
+    if (end < 0) end = 0;
+  } else {
+    if (end > length) end = length;
+  }
+
+  for (; i < end; i++)
+    array[i] = value;
+  return array;
+}
 endmacro
 
 TYPED_ARRAYS(TYPED_ARRAY_HARMONY_ADDITIONS)
 
 
 function HarmonyTypedArrayExtendPrototypes() {
 macro EXTEND_TYPED_ARRAY(ARRAY_ID, NAME, ELEMENT_SIZE)
   %CheckIsBootstrapping();
 
   // Set up non-enumerable functions on the prototype object.
   InstallFunctions(global.NAME.prototype, DONT_ENUM, $Array(

[wingo, 2014/11/12 09:29:56]

Both of these should be DONT_WRITE and DONT_DELETE as well.

[aperez, 2014/11/13 16:34:36]

Acknowledged.

-    "forEach", NAMEForEach
+    "forEach", NAMEForEach,
+    "fill", NAMEFill
   ));
 endmacro
 
   TYPED_ARRAYS(EXTEND_TYPED_ARRAY)
 }
 
 HarmonyTypedArrayExtendPrototypes();