Makes sure "adding" a string to a Callbacks object doesn't cause a stack...

Makes sure "adding" a string to a Callbacks object doesn't cause a stack overflow, just ignore the value like 1.7.x righfully did. Fixes #12233. Unit tests added.

src/callbacks.js

@@ -92,9 +92,10 @@ jQuery.Callbacks = function( options ) {
 					var start = list.length;
 					(function add( args ) {
 						jQuery.each( args, function( _, arg ) {
-							if ( jQuery.isFunction( arg ) && ( !options.unique || !self.has( arg ) ) ) {
+							var type = jQuery.type( arg );
+							if ( type === "function" && ( !options.unique || !self.has( arg ) ) ) {
 								list.push( arg );
-							} else if ( arg && arg.length ) {
+							} else if ( arg && arg.length && type !== "string" ) {
 								// Inspect recursively
 								add( arg );
 							}

test/unit/callbacks.js

@@ -250,3 +250,12 @@ test( "jQuery.Callbacks.remove - should remove all instances", function() {
 		ok( true, "end of test" );
 	}).remove( fn ).fire();
 });
+
+test( "jQuery.Callbacks() - adding a string doesn't cause a stack overflow", function() {
+
+	expect( 1 );
+
+	jQuery.Callbacks().add( "hello world" );
+
+	ok( true, "no stack overflow" );
+});