How to negate code in "if" statement block in JavaScript -JQuery like 'if not then..'
You can use the Logical NOT ! operator:
if (!$(this).parent().next().is('ul')){
Or equivalently (see comments below):
if (! ($(this).parent().next().is('ul'))){
For more information, see the Logical Operators section of the MDN docs.
Try negation operator ! before $(this):
if (!$(this).parent().next().is('ul')){