Fix #12411, .removeClass(undefined) is a chaining no-op. Close gh-913.

.removeClass() //removes all classes, as documented
.removeClass(window.nonExistentVariable) // removes nothing

src/attributes.js

@@ -78,7 +78,7 @@ jQuery.fn.extend({
 				jQuery( this ).removeClass( value.call(this, j, this.className) );
 			});
 		}
-		if ( (value && typeof value === "string") || value === undefined ) {
+		if ( (value && typeof value === "string") || !arguments.length ) {
 			removes = ( value || "" ).split( core_rspace );
 
 			for ( i = 0, l = this.length; i < l; i++ ) {

test/unit/attributes.js

@@ -1202,6 +1202,15 @@ test( "removeClass() removes duplicates", function() {
 	ok( !$div.hasClass("x"), "Element with multiple same classes does not escape the wrath of removeClass()" );
 });
 
+test("removeClass(undefined) is a no-op", function() {
+	expect( 1 );
+
+	var $div = jQuery("<div class='base second'></div>");
+	$div.removeClass( undefined );
+
+	ok( $div.hasClass("base") && $div.hasClass("second"), "Element still has classes after removeClass(undefined)" );
+});
+
 var testToggleClass = function(valueObj) {
 	expect( 17 );