Core: Simplify isPlainObject

Fixes gh-2986
Close gh-2998

src/core.js

@@ -1,6 +1,7 @@
 define( [
 	"./var/arr",
 	"./var/document",
+	"./var/getProto",
 	"./var/slice",
 	"./var/concat",
 	"./var/push",
@@ -8,10 +9,13 @@ define( [
 	"./var/class2type",
 	"./var/toString",
 	"./var/hasOwn",
+	"./var/fnToString",
+	"./var/ObjectFunctionString",
 	"./var/support",
 	"./core/DOMEval"
-], function( arr, document, slice, concat,
-	push, indexOf, class2type, toString, hasOwn, support, DOMEval ) {
+], function( arr, document, getProto, slice, concat, push, indexOf,
+	class2type, toString, hasOwn, fnToString, ObjectFunctionString,
+	support, DOMEval ) {
 
 var
 	version = "@VERSION",
@@ -225,28 +229,24 @@ jQuery.extend( {
 	},
 
 	isPlainObject: function( obj ) {
-		var key;
+		var proto, Ctor;
 
-		// Not plain objects:
-		// - Any object or value whose internal [[Class]] property is not "[object Object]"
-		// - DOM nodes
-		// - window
-		if ( jQuery.type( obj ) !== "object" || obj.nodeType || jQuery.isWindow( obj ) ) {
+		// Detect obvious negatives
+		// Use toString instead of jQuery.type to catch host objects
+		if ( !obj || toString.call( obj ) !== "[object Object]" ) {
 			return false;
 		}
 
-		// Not own constructor property must be Object
-		if ( obj.constructor &&
-				!hasOwn.call( obj, "constructor" ) &&
-				!hasOwn.call( obj.constructor.prototype || {}, "isPrototypeOf" ) ) {
-			return false;
+		proto = getProto( obj );
+
+		// Objects with no prototype (e.g., `Object.create( null )`) are plain
+		if ( !proto ) {
+			return true;
 		}
 
-		// Own properties are enumerated firstly, so to speed up,
-		// if last one is own, then all properties are own
-		for ( key in obj ) {}
-
-		return key === undefined || hasOwn.call( obj, key );
+		// Objects with prototype are plain iff they were constructed by a global Object function
+		Ctor = hasOwn.call( proto, "constructor" ) && proto.constructor;
+		return typeof Ctor === "function" && fnToString.call( Ctor ) === ObjectFunctionString;
 	},
 
 	isEmptyObject: function( obj ) {

src/var/ObjectFunctionString.js

@@ -0,0 +1,5 @@
+define( [
+	"./fnToString"
+], function( fnToString ) {
+	return fnToString.call( Object );
+} );

src/var/fnToString.js

@@ -0,0 +1,5 @@
+define( [
+	"./hasOwn"
+], function( hasOwn ) {
+	return hasOwn.toString;
+} );

src/var/getProto.js

@@ -0,0 +1,3 @@
+define( function() {
+	return Object.getPrototypeOf;
+} );