I recently ran into a situation where I realized that the nice slick J-Query styling that I’d given to my select menus hadn’t carried over to my dynamically added fields.  This is because, my javascript ran $('select').customSelect(); when the page loaded but at no point after. I therefore needed to figure out how to trigger that function once a dynamic field was added.

After quite a bit of searching around (on the order of hours) and several dead ends, I finally learned that cocoon has many different actions that can be used with on() In my case, all I needed was the following:
$('#test').on("cocoon:after-insert", function(e, added_item){
added_item.find('select').customSelect();
});


And I was good to go.