Core: Tiny efficiency fix to jQuery.extend / jQuery.fn.extend (#4246)

Read target[name] only when it's needed.

In addition to doing the property read-only when needed, this
avoids a slow path in V8 (see the issue for more details).

Fixes gh-4245
Closes gh-4246

src/core.js

@@ -156,7 +156,6 @@ jQuery.extend = jQuery.fn.extend = function() {
 
 			// Extend the base object
 			for ( name in options ) {
-				src = target[ name ];
 				copy = options[ name ];
 
 				// Prevent never-ending loop
@@ -167,14 +166,17 @@ jQuery.extend = jQuery.fn.extend = function() {
 				// Recurse if we're merging plain objects or arrays
 				if ( deep && copy && ( jQuery.isPlainObject( copy ) ||
 					( copyIsArray = Array.isArray( copy ) ) ) ) {
+					src = target[ name ];
 
-					if ( copyIsArray ) {
-						copyIsArray = false;
-						clone = src && Array.isArray( src ) ? src : [];
-
+					// Ensure proper type for the source value
+					if ( copyIsArray && !Array.isArray( src ) ) {
+						clone = [];
+					} else if ( !copyIsArray && !jQuery.isPlainObject( src ) ) {
+						clone = {};
 					} else {
-						clone = src && jQuery.isPlainObject( src ) ? src : {};
+						clone = src;
 					}
+					copyIsArray = false;
 
 					// Never move original objects, clone them
 					target[ name ] = jQuery.extend( deep, clone, copy );