Question

If #box doesn’t have a containing div with a class of current-map, then do something.

I thought the ! would have worked but it doesn’t.

if (!$('#box').closest('.current-map')) {

     //do something

}

Answer 1

You could check the length of the resulting selection. If it’s 0, there was no ancestor .current-map element:

if($("#box").closest(".current-map").length) {
    //.current-map is an ancestor
}

Your attempt does not work because most jQuery methods will return an instance of jQuery whether it contains any DOM elements or not, and that will always evaluate to true.