Due to this line; ``` arrayMode = $([target.toLowerCase(), 'default']).filter(['default','concat','replace','extend'])[0]; ``` (https://github.com/mistic100/jQuery.extendext/blob/master/jQuery.extendext.js#L40) In jQuery < 3.0.0 this returns at least `"default"`, in jQuery > 3.0.0 it always returns `undefined`.