JavaScript: How do I create JSONP?

It is simple. Simply accept a parameter called callback in the GET.

Then wrap the callback JavaScript function around your data.

Example in PHP:

<?php

$data = '{}'; // json string

if(array_key_exists('callback', $_GET)){

    header('Content-Type: text/javascript; charset=utf8');
    header('Access-Control-Allow-Origin: http://www.example.com/');
    header('Access-Control-Max-Age: 3628800');
    header('Access-Control-Allow-Methods: GET, POST, PUT, DELETE');

    $callback = $_GET['callback'];
    echo $callback.'('.$data.');';

}else{
    // normal JSON string
    header('Content-Type: application/json; charset=utf8');

    echo $data;
}

It's idea is to simply return a JavaScript file which calls the callback function with the JSON object as the first parameter of the JavaScript callback function.

You can use the built-in json_encode() function to create JSON strings (which $data in our example above contains) from arrays and objects in PHP.

To use the JSONP service, you can use the <script> tag:

<script>
    function receiver(data){
        console.log(data);
    }
</script>
<script src="data-service.php?callback=receiver"></script>

You need a server-side language, the callback parameter is simply a GET parameter, you read the param, and you wrap the JSON response into a function call and you print it like this callback(jsonResponse);.

I leave you a really minimalist example using Python since you don't mention any server-side language:

import os
import cgi

form = cgi.FieldStorage()
callback = form.getvalue('callback','')

address = cgi.escape(os.environ["REMOTE_ADDR"])

json = '{"ip": "'+address+'", "address":"'+address+'"}'

#Allow cross domain XHR
print 'Access-Control-Allow-Origin: *'
print 'Access-Control-Allow-Methods: GET'

if callback != '':
  print 'Content-Type: application/javascript'
  result = callback+'('+json+');'
else:
  print 'Content-Type: application/json'
  result = json

print ''
print result

That is the code of a small JSONP service used to retrieve the client IP address made by Zach and it is hosted on the Google App Engine.

Mauris already gave you a working example. I would only add that you should check if a callback param is present and non-empty, and if not, return the json data as is without the parentheses. So basically your api will be JSON with provision of being JSON-P when callback is given.

To consume the JSON-P webservice, unless you use a framework like YUI or jQuery, you can simply create a script node dynamically and set its src attribute to point to the webservice. Remember to remove the node from the dom before repeating it again, since this dynamic script node is single use only.

Easy with jQuery, that is the client side:

  $.ajax({
    	dataType: 'jsonp',
    	data: "somedata="+somevalue,
    	//this is very important since it's the callback we will and that allow cross domain
    	jsonp: 'jsonp_callback',
    	url: 'http://example2.com',
    	//function we trigger on success
    	success: ParseJson
         //error handling not working with jsonP
    	 //error: handleError
    	});

function ParseJson(data)
{
for (var key in data) {
  if (data.hasOwnProperty(key)) {
    alert(key + " -> " + data[key]);
  }
}
}

And be sure that you get proper json from the server side;
and don't forget to return the jsonp_callback param, otherwise it won't work!!!!!
and that's it really.

// Adds script tag to head of the page
function addScriptToHead(source, code, type) {
    var script = document.createElement('script');
    if (type === 'js') {
        script.setAttribute('type', 'text/javascript');
    }
    if (source !== '') {
        script.setAttribute('src', source);
    }
    if (code !== '') {
        if (document.all && !window.opera)  {
            script.text = code;
        } else {
            script.innerHTML = code;
        }
    }
    document.getElementsByTagName('head')[0].appendChild(script);
}


// Callback function
function addScriptToHead(any_param) {

// do whatever needs to be done

}

//call example

addScriptToHead('http://url_to_receiver_script/index.php&param=anything', '', 'js');

/// the callback script should return name of the Callback function, i.e. if you type in browser

http://url_to_receiver_script/index.php&param=anything

it should return just a text (name of existing processing function): addScriptToHead(any_param)

works like a clock in any browser.

example here http://www.insideria.com/2009/03/what-in-the-heck-is-jsonp-and.html basically

<script src=".../example2...?output=json;callback=loadit"></script>
<script>
alert( "I got this from example2 " + loadit);
</script>