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JSON output

The JSON standard (JavaScript Object Notation) is an open, text-based data format, designed for human-readable data interchange. Despite its descendance from JavaScript - you need JavaScript to process it, XSLT can't be used - it is language-independent. JSON is more lightweight than XML (about half the data) and therefore faster to process. Another advantage is that JSON is allowed in cross domain AJAX requests.
More general information about JSON can be found here:

A number of Adlib API commands have two optional JSON-specific arguments available: output and callback.

output

output determines the output format to be returned. Besides XML, the output format JSON can be returned by wwwopac.ashx. JSON output is smaller than XML, so it may improve performance of your API application. Values for output can be either json or xml (xml is the default, so you don't need to specify that explicitly).
Although JSON is not XML, output=json can be used together with the xmltype argument: if xmltype is set to grouped, the JSON output will be grouped, and for xmltype=unstructured the JSON output will be unstructured.
The output format produced by jQuery applications is always JSON, so you don't need to specify the output type there.

callback

callback returns JSON output wrapped in brackets with your callback function name, for AJAX requests: <callback function name>( <JSON output> ); You don't need to specify output=json, because the output format is JSON by default.