Question

I’m creating an iOS / Droid app using AJAX, jQuery, and Phonegap. The backend is a Drupal 7 site serving content via web services. I’m a little confused about the necessity for a JSONP callback, though.

My understanding is that JSONP is required in order to allow cross domain requests. And, in part, it works by wrapping the JSON data within a callback function, thus making it an object.

However, what I’m finding is that I can send the JSONP request without a callback, and parse the data via jQuery without the callback function. So the JSONP data looks exactly like JSON data.

Am I missing something? If providing the data as function is one of the benefits of JSONP, shouldn’t it be required at some stage in the process? Am I bypassing some level of security by not sending a callback parameter?

Thanks!

FYI, here is the jQuery AJAX code:

$.ajax({
    type: 'GET',
    cache: false,
    url: 'http://www.example.com/mobile/menu/devel-gosto.jsonp',
    dataType: 'jsonp',
    timeout: menuLoadTimeout,
    success: function (data) {
        menu = populateSlider(data.tree, 0);
    }
}); 

So I’m setting the dataType to ‘jsonp’, calling to a web service that returns JSONP data, but leaving the callback out of the URL. The web service responds without a callback wrapper. And everything works fine.

Answer 1

A JSONP call doesn’t work without a callback. The data is loaded in a script tag, and if the code is not in a form of a method call, the result would just be an object that was discarded, and the success callback method would never be called.

The ajax method is adding a callback parameter to the URL even if you don’t specify one.

In the documentation, under the "jsonp" value for the dataType setting:

“Adds an extra “?callback=?” to the end of your URL to specify the
callback.”

http://api.jquery.com/jQuery.ajax/