Core: restore enumeration behavior in isPlainObject

Fixes gh-2968
Close gh-2970

1 files changed, 6 insertions, 3 deletions

diff --git a/src/core.js b/src/core.js
index b10128c7a..17673dfac 100644
--- a/src/core.js
+++ b/src/core.js
@@ -225,6 +225,7 @@ jQuery.extend( {
 	},
 
 	isPlainObject: function( obj ) {
+		var key;
 
 		// Not plain objects:
 		// - Any object or value whose internal [[Class]] property is not "[object Object]"
@@ -239,9 +240,11 @@ jQuery.extend( {
 			return false;
 		}
 
-		// If the function hasn't returned already, we're confident that
-		// |obj| is a plain object, created by {} or constructed with new Object
-		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 );
 	},
 
 	isEmptyObject: function( obj ) {