callback function parsing JSON in JQuery -

i'm new jquery , maybe n00b question. , english not best.

i wrote service in google app engine application delivers data in json format, works ok, wasn't able parse json data using jquery:

 var url= 'myapp.appspot.com/myservice.json?someparams'; $.getjson(url, function(json){     alert("success parsing json");  // never reached code      .... }); 

after few days of reading posts , tutorials felt slideshare: http://www.slideshare.net/andymckay/cross-domain-webmashups-with-jquery-and-google-app-engine

while reading slide 23 noticed "callback=?" parameter , tried code in slide 42:

 class myjsonhandler(webapp.requesthandler):     def get(self):         ## retrieve data db or memcached         jsondata = json.dumps(data)         if self.request.get('callback'):             self.response.out.write('%s(%s)' % (self.request.get('callback'), jsondata))         else:             self.response.out.write(jsondata) 

and in jquery function:

 $.getjson(url+'&callback=?', function(json){     alert("success parsing json");  // i'm here !!     .... }); 

my question is:

why "callback" parameter necessary make work? difference '?("myjson": [{"a-lot" : "of-data"}])' makes??

thanks all.

the callback parameter used implement jsonp.

jquery's getjson method creates <script> tag point url give it.
url expected return call function specified in callback parameter, passing data parameter.

unlike normal ajax requests, jsonp requests can made across domains.