jQuery .nextOrFirst()

gistfile1.js

$.fn.nextOrFirst = function(selector){
    var next = this.next(selector);
    return (next.length) ? next : this.prevAll(selector).last();
};

$.fn.prevOrLast = function(selector){
    var prev = this.prev(selector);
    return (prev.length) ? prev : this.nextAll(selector).last();
};

Good thing you saved this. The source seems to have gone offline. Thanks!