JSON callbacks

Web scripts that provide JSON responses are often invoked directly from within a web browser via the XMLHttpRequest object. This is a technique popularly known as AJAX. For security reasons, solutions like these may run into cross-domain issues, a restriction that requires you to proxy your requests on the server side. Typically, to work around these issues, public services, such as Yahoo! JSON Services, provide a callback mechanism.

A full description of the JSON callback mechanism can be found at http://developer.yahoo.com/common/json.html#callbackparam on the Yahoo! Developer Network.

Web scripts also provide this mechanism, which wraps the JSON response text in parentheses and a function name of your choosing. A callback is invoked by adding the following URL query parameter to the web script request:

alf_callback=<function>

The function parameter specifies the name of a client-side JavaScript function to invoke.

Using the JSON callback
Creating a callback example simply involves creating an HTML page that invokes the Hello User web script with a callback that displays the JSON response in an alert box.
Understanding how the JSON callback works
The easiest way to understand the callback example is to invoke the Hello User web script directly and interrogate the response.