A typical prefilter registration using $.ajaxPrefilter() looks like this:

$.ajaxPrefilter( function( options, originalOptions, jqXHR ) {
  // Modify options, control originalOptions, store jqXHR, etc
});

where:

• options are the request options
• originalOptions are the options as provided to the ajax method, unmodified and, thus, without defaults from ajaxSettings
• jqXHR is the jqXHR object of the request

Prefilters are a perfect fit when custom options need to be handled. Given the following code, for example, a call to $.ajax() would automatically abort a request to the same URL if the custom abortOnRetry option is set to true:

var currentRequests = {};

$.ajaxPrefilter(function( options, originalOptions, jqXHR ) {
  if ( options.abortOnRetry ) {
    if ( currentRequests[ options.url ] ) {
      currentRequests[ options.url ].abort();
    }
    currentRequests[ options.url ] = jqXHR;
  }
});

Prefilters can also be used to modify existing options. For example, the following proxies cross-domain requests through http://mydomain.net/proxy/:

$.ajaxPrefilter( function( options ) {
  if ( options.crossDomain ) {
    options.url = "http://mydomain.net/proxy/" + encodeURIComponent( options.url );
    options.crossDomain = false;
  }
});

If the optional dataTypes argument is supplied, the prefilter will be only be applied to requests with the indicated dataTypes. For example, the following only applies the given prefilter to JSON and script requests:

$.ajaxPrefilter( "json script", function( options, originalOptions, jqXHR ) {
  // Modify options, control originalOptions, store jqXHR, etc
});

The $.ajaxPrefilter() method can also redirect a request to another dataType by returning that dataType. For example, the following sets a request as "script" if the URL has some specific properties defined in a custom isActuallyScript() function:

$.ajaxPrefilter(function( options ) {
  if ( isActuallyScript( options.url ) ) {
    return "script";
  }
});

This would ensure not only that the request is considered "script" but also that all the prefilters specifically attached to the script dataType would be applied to it.