Object.observe: avoid accessing acceptList properties more than once

src/object-observe.js

 // Copyright 2012 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";
 
 // Overview:
 //
 // This file contains all of the routing and accounting for Object.observe.
 // User code will interact with these mechanisms via the Object.observe APIs
@@ skipping 109 matching lines @@
 }
 
 function TypeMapAddType(typeMap, type, ignoreDuplicate) {
   typeMap[type] = ignoreDuplicate ? 1 : (typeMap[type] || 0) + 1;
 }
 
 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;
 }
 
 function TypeMapHasType(typeMap, type) {
   return !!typeMap[type];
 }
 
 function TypeMapIsDisjointFrom(typeMap1, typeMap2) {
   if (!typeMap1 || !typeMap2)
     return true;
 
   for (var type in typeMap1) {
     if (TypeMapHasType(typeMap1, type) && TypeMapHasType(typeMap2, type))
       return false;
   }
 
   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
 // set for Object.observe, the observer is represented as a direct reference
 // to the callback. An observer never changes its accept types and thus never
 // needs to "normalize".
 function ObserverCreate(callback, acceptList) {
   if (IS_UNDEFINED(acceptList))
     return callback;
   var observer = nullProtoObject();
   observer.callback = callback;
-  observer.accept = TypeMapCreateFromList(acceptList);
+  observer.accept = acceptList;
   return observer;
 }
 
 function ObserverGetCallback(observer) {
   return IS_SPEC_FUNCTION(observer) ? observer : observer.callback;
 }
 
 function ObserverGetAcceptTypes(observer) {
   return IS_SPEC_FUNCTION(observer) ? defaultAcceptTypes : observer.accept;
 }
@@ skipping 115 matching lines @@
 
 function ObjectInfoRemovePerformingType(objectInfo, type) {
   objectInfo.performingCount--;
   TypeMapRemoveType(objectInfo.performing, type);
 }
 
 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
 // priority. When a change record must be enqueued for the callback, it
 // normalizes. When delivery clears any pending change records, it re-optimizes.
 function CallbackInfoGet(callback) {
   return GetCallbackInfoMap().get(callback);
 }
 
 function CallbackInfoGetOrCreate(callback) {
@@ skipping 26 matching lines @@
 
 function ObjectObserve(object, callback, acceptList) {
   if (!IS_SPEC_OBJECT(object))
     throw MakeTypeError("observe_non_object", ["observe"]);
   if (%IsJSGlobalProxy(object))
     throw MakeTypeError("observe_global_proxy", ["observe"]);
   if (!IS_SPEC_FUNCTION(callback))
     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;
 }
 
 function ObjectUnobserve(object, callback) {
   if (!IS_SPEC_OBJECT(object))
     throw MakeTypeError("observe_non_object", ["unobserve"]);
   if (%IsJSGlobalProxy(object))
     throw MakeTypeError("observe_global_proxy", ["unobserve"]);
   if (!IS_SPEC_FUNCTION(callback))
     throw MakeTypeError("observe_non_function", ["unobserve"]);
@@ skipping 242 matching lines @@
     "observe", ArrayObserve,
     "unobserve", ArrayUnobserve
   ));
   InstallFunctions(notifierPrototype, DONT_ENUM, $Array(
     "notify", ObjectNotifierNotify,
     "performChange", ObjectNotifierPerformChange
   ));
 }
 
 SetupObjectObserve();