Array.observe emit splices for array length change and update index >= length

src/v8natives.js

Index: src/v8natives.js
diff --git a/src/v8natives.js b/src/v8natives.js
index f5ab122ff0aa8d2a1604f640ca97de7cb2499d1e..d444613f3a8ad0422aeae6d84c0f7cf7636bc2e8 100644
--- a/src/v8natives.js
+++ b/src/v8natives.js
@@ -873,6 +873,7 @@ function DefineArrayProperty(obj, p, desc, should_throw) {
   // Step 3 - Special handling for length property.
   if (p === "length") {
     var length = obj.length;
+    var old_length = length;
     if (!desc.hasValue()) {
       return DefineObjectProperty(obj, "length", desc, should_throw);
     }
@@ -889,8 +890,24 @@ function DefineArrayProperty(obj, p, desc, should_throw) {
       }
     }
     var threw = false;
+
+    var emit_splice = %IsObserved(obj) && new_length !== old_length;
+    var removed;
+    if (emit_splice) {
+      BeginPerformSplice(obj);
+      removed = [];
+      if (new_length < old_length)
+        removed.length = old_length - new_length;
+    }
+
     while (new_length < length--) {
-      if (!Delete(obj, ToString(length), false)) {
+      var index = ToString(length);
+      if (emit_splice) {
+        var deletedDesc = GetOwnProperty(obj, index);
+        if (deletedDesc && deletedDesc.hasValue())
+          removed[length - new_length] = deletedDesc.getValue();
+      }
+      if (!Delete(obj, index, false)) {
         new_length = length + 1;
         threw = true;
         break;
@@ -902,13 +919,18 @@ function DefineArrayProperty(obj, p, desc, should_throw) {
     // respective TODO in Runtime_DefineOrRedefineDataProperty.
     // For the time being, we need a hack to prevent Object.observe from
     // generating two change records.
-    var isObserved = %IsObserved(obj);
-    if (isObserved) %SetIsObserved(obj, false);
     obj.length = new_length;
     desc.value_ = void 0;
     desc.hasValue_ = false;
     threw = !DefineObjectProperty(obj, "length", desc, should_throw) || threw;
-    if (isObserved) %SetIsObserved(obj, true);
+    if (emit_splice) {
+      EndPerformSplice(obj);
+      EnqueueSpliceRecord(obj,
+          new_length < old_length ? new_length : old_length,
+          removed,
+          removed.length,
+          new_length > old_length ? new_length - old_length : 0);
+    }
     if (threw) {
       if (should_throw) {
         throw MakeTypeError("redefine_disallowed", [p]);
@@ -916,27 +938,24 @@ function DefineArrayProperty(obj, p, desc, should_throw) {
         return false;
       }
     }
-    if (isObserved) {
-      var new_desc = GetOwnProperty(obj, "length");
-      var updated = length_desc.value_ !== new_desc.value_;
-      var reconfigured = length_desc.writable_ !== new_desc.writable_ ||
-          length_desc.configurable_ !== new_desc.configurable_ ||
-          length_desc.enumerable_ !== new_desc.configurable_;
-      if (updated || reconfigured) {
-        NotifyChange(reconfigured ? "reconfigured" : "updated",
-            obj, "length", length_desc.value_);
-      }
-    }
     return true;
   }
 
   // Step 4 - Special handling for array index.
   var index = ToUint32(p);
+  var emit_splice = false;
   if (ToString(index) == p && index != 4294967295) {
     var length = obj.length;
+    if (index >= length && %IsObserved(obj)) {
+      emit_splice = true;
+      BeginPerformSplice(obj);
+    }
+
     var length_desc = GetOwnProperty(obj, "length");
     if ((index >= length && !length_desc.isWritable()) ||
         !DefineObjectProperty(obj, p, desc, true)) {
+      if (emit_splice)
+        EndPerformSplice(obj);
       if (should_throw) {
         throw MakeTypeError("define_disallowed", [p]);
       } else {
@@ -946,6 +965,10 @@ function DefineArrayProperty(obj, p, desc, should_throw) {
     if (index >= length) {
       obj.length = index + 1;
     }
+    if (emit_splice) {
+      EndPerformSplice(obj);
+      EnqueueSpliceRecord(obj, length, [], 0, index + 1 - length);
+    }
     return true;
   }