Question

I can’t understand the doc of the jQuery.ajax, specifically two options: jsonp and jsonpCallback, so can somebody be so pleasant to explain?

What I do understand is that jsonp is a name of a GET parameter which server expects (usually ‘callback’) and jsonpCallback is a name of a function to wrap a response. Seems simple.

But the explanation at the jQuery.ajax doc makes this a bit complicated. I would like to cite the complete text for jsonp option here and mark with bold what is obscure to me:

jsonp

Override the callback function name in a jsonp request. This value
will be used instead of ‘callback’ in the ‘callback=?’ part of the
query string in the url. So {jsonp:’onJSONPLoad’} would result in
‘onJSONPLoad=?’ passed to the server. As of jQuery 1.5, setting the
jsonp option to false prevents jQuery from adding the “?callback”
string to the URL or attempting to use “=?” for transformation. In
this case, you should also explicitly set the jsonpCallback setting.
For example, { jsonp: false, jsonpCallback: “callbackName” }

So the questions are:

1.What does it mean “=?” or ‘callback=?’ (extra question mark)? When I perform a JSONP AJAX-request like here:

 $.ajax('http://fake.com',{
     dataType: 'jsonp',
     success: function(data) {console.log(data);}
 }); 

The URL looks like this, there is no such question mark:

http://fake.com/?callback=jQuery18104830878316494931_1352981996209&_=1352981999411

2.What is the extra parameter (underscore) _=1352981999411?

3.What do the words In this case, you should also explicitly set the jsonpCallback setting mean? I can’t see any interrelationship. If I set { jsonp: false, jsonpCallback: "callbackName" }, as it is said in the doc the query will look like this:

http://fake.com/?_=1352981999411

There is no use of the “callbackName” at all so why to specify it?

My appreciation.

Answer 1

The callback name is optional, if you don’t supply one it uses a default one. Hence, if you leave out onJsonLoad you just get callback.

What happens is the JSON is wrapped with a function, because JSONP works through the <script> tag, so your normal JSON:

{prop: value}

Becomes:

callback({prop: value});

When the script tag is loaded jQuery can then call this function and get back the JSON data.

The _=123456788 is just a Date.getTime() to stop the request from caching.