Object.observe: avoid accessing acceptList properties more than once

src/object-observe.js

Index: src/object-observe.js
diff --git a/src/object-observe.js b/src/object-observe.js
index 532b0d25254c28dfee157e61da3e55c175861002..f4b1f539634a58f583265695ed59e92c72f26f51 100644
--- a/src/object-observe.js
+++ b/src/object-observe.js
@@ -127,9 +127,9 @@ function TypeMapRemoveType(typeMap, type) {
   typeMap[type]--;
 }
 
-function TypeMapCreateFromList(typeList) {
+function TypeMapCreateFromList(typeList, length) {
   var typeMap = TypeMapCreate();
-  for (var i = 0; i < typeList.length; i++) {
+  for (var i = 0; i < length; i++) {
     TypeMapAddType(typeMap, typeList[i], true);
   }
   return typeMap;
@@ -151,14 +151,17 @@ function TypeMapIsDisjointFrom(typeMap1, typeMap2) {
   return true;
 }
 
-var defaultAcceptTypes = TypeMapCreateFromList([
-  'add',
-  'update',
-  'delete',
-  'setPrototype',
-  'reconfigure',
-  'preventExtensions'
-]);
+var defaultAcceptTypes = (function() {
+  var defaultTypes = [
+    'add',
+    'update',
+    'delete',
+    'setPrototype',
+    'reconfigure',
+    'preventExtensions'
+  ];
+  return TypeMapCreateFromList(defaultTypes, defaultTypes.length);
+})();
 
 // An Observer is a registration to observe an object by a callback with
 // a given set of accept types. If the set of accept types is the default
@@ -170,7 +173,7 @@ function ObserverCreate(callback, acceptList) {
     return callback;
   var observer = nullProtoObject();
   observer.callback = callback;
-  observer.accept = TypeMapCreateFromList(acceptList);
+  observer.accept = acceptList;
   return observer;
 }
 
@@ -306,16 +309,18 @@ function ObjectInfoGetPerformingTypes(objectInfo) {
   return objectInfo.performingCount > 0 ? objectInfo.performing : null;
 }
 
-function AcceptArgIsValid(arg) {
+function ConvertAcceptListToTypeMap(arg) {
+  // We use undefined as a sentinel for the default accept list.
   if (IS_UNDEFINED(arg))
-    return true;
+    return arg;
 
-  if (!IS_SPEC_OBJECT(arg) ||
-      !IS_NUMBER(arg.length) ||
-      arg.length < 0)
-    return false;
+  if (!IS_SPEC_OBJECT(arg))
+    throw MakeTypeError("observe_accept_invalid");
 
-  return true;
+  var len = ToInteger(arg.length);
+  if (len < 0) len = 0;
+
+  return TypeMapCreateFromList(arg, len);
 }
 
 // CallbackInfo's optimized state is just a number which represents its global
@@ -362,15 +367,14 @@ function ObjectObserve(object, callback, acceptList) {
     throw MakeTypeError("observe_non_function", ["observe"]);
   if (ObjectIsFrozen(callback))
     throw MakeTypeError("observe_callback_frozen");
-  if (!AcceptArgIsValid(acceptList))
-    throw MakeTypeError("observe_accept_invalid");
 
   return %ObjectObserveInObjectContext(object, callback, acceptList);
 }
 
 function NativeObjectObserve(object, callback, acceptList) {
   var objectInfo = ObjectInfoGetOrCreate(object);
-  ObjectInfoAddObserver(objectInfo, callback, acceptList);
+  var typeList = ConvertAcceptListToTypeMap(acceptList);
+  ObjectInfoAddObserver(objectInfo, callback, typeList);
   return object;
 }