Implement ES6 Array.prototype.toString behind --harmony-tostring

src/harmony-tostring.js

Index: src/harmony-tostring.js
diff --git a/src/harmony-tostring.js b/src/harmony-tostring.js
index 0336456bb2827c13c75bd5f823731a2223e65540..e36ef0d111d7e4015a16327e4b4ef0ceb05d296a 100644
--- a/src/harmony-tostring.js
+++ b/src/harmony-tostring.js
@@ -33,13 +33,25 @@ function ObjectToStringHarmony() {
   if (IS_UNDEFINED(tag)) {
     tag = builtinTag;
   } else if (!IS_STRING(tag)) {
-    return "[object ???]"
+    return "[object ???]";
   } else if (tag !== builtinTag && kBuiltinStringTags[tag]) {
     return "[object ~" + tag + "]";
   }
   return "[object " + tag + "]";
 }
 
+// ES6 draft 12-06-14, section 22.1.3.27
+function ArrayToStringHarmony() {
+  if (IS_ARRAY(this) && this.join === ArrayJoin) {

[arv (Not doing code reviews), 2015/01/12 15:46:40]

It looks like we are doing Get(object, 'join') twice?

+    return Join(this, this.length, ',', ConvertToString);
+  }
+
+  var O = ToObject(this);
+  var join = O.join;
+  if (IS_SPEC_FUNCTION(join)) return %_CallFunction(O, join);
+  return %_CallFunction(O, ObjectToStringHarmony);

[arv (Not doing code reviews), 2015/01/12 15:46:40]

Could we just change ArrayToString to use DefaultObjectToString instead of
duplicating ArrayToString?

[caitp (gmail), 2015/01/12 15:53:16]

hmm, that's a good point

+}
+
 function HarmonyToStringExtendSymbolPrototype() {
   %CheckIsBootstrapping();
 
@@ -70,3 +82,23 @@ function HarmonyToStringExtendObjectPrototype() {
 }
 
 HarmonyToStringExtendObjectPrototype();
+
+function HarmonyToStringExtendArrayPrototype() {
+  %CheckIsBootstrapping();
+
+  // Can't use InstallFunctions() because will fail in Debug mode.
+  // Emulate InstallFunctions here.
+  %FunctionSetName(ArrayToStringHarmony, "toString");
+  %FunctionRemovePrototype(ArrayToStringHarmony);
+  %SetNativeFlag(ArrayToStringHarmony);
+
+  // Set up the non-enumerable functions on the Array prototype object.
+  var desc = ToPropertyDescriptor({
+    value: ArrayToStringHarmony
+  });
+  DefineOwnProperty($Array.prototype, "toString", desc, false);

[arv (Not doing code reviews), 2015/01/12 15:46:40]

DefineOwnProperty takes a descriptor... can we use a lower level API (above too)
that does not require that temporary descriptor object?

[caitp (gmail), 2015/01/12 15:53:16]

Sort of --- Runtime_AddOwnProperty (used by InstallFunctions) will throw if the
property already exists (in debug mode only).

There's a `RUNTIME_ASSERT(!it.IsFound());` in there which causes that, I'm not
sure if it's a good idea to remove it

+
+  %ToFastProperties($Array.prototype);
+}
+
+HarmonyToStringExtendArrayPrototype();