Agree, “callback” works too and is arguably better as it is just as general and intuitive. Downside is it can’t be inlined as an anonymous function, so it may entail a bit more JS code than some JSONP usages, which may make people want “variable” too, which starts to get complex and error-prone for API providers, which is the argument for JSONP :). If I were implementing an API, I would probably provide all three, as they can happily co-exist.

JSONP is a simple protocol where there’s a single relevant parameter in the URL, called precisely “jsonp”. The idea is to be as general as possible. Using JSONP, it is possible to specify a callback or a variable or anything else you could imagine (e.g. you could pass in “jsonp” param as “el.style.backgroundColor=” to affect an element’s appearance).

http://ajaxian.com/archives/jsonp-json-with-padding

 def show
    @grocery = Grocery.find(params[:id])
	
    respond_to do |format|
        format.html # falls back on the rhtml file
        format.js { render_json @grocery.to_json }
    end
  end