jquery.jsonpCallback.js

jQuery.useJsonpCallback = (jQuery.support.jsonpCallback = parseFloat(jQuery.fn.jquery) >= 1.4) ? jQuery.noop : function () {
  var cfg = this, jsonp = cfg.dataType === 'script' && cfg.jsonpCallback;
  if (jsonp && !window[jsonp]) {
    window[jsonp] = function (data) {
      cfg.success.call(cfg, data, 'success');
      try {
        delete window[jsonp];
      } catch (ie6) {
        // cannot delete window properties in IE6
        window[jsonp] = undefined; 
      }
    };
  }
};

jquery.jsonpCallback.min.js

jQuery.useJsonpCallback=(jQuery.support.jsonpCallback=parseFloat(jQuery.fn.jquery)>=1.4)?jQuery.noop:function(){var a=this,b=a.dataType==="script"&&a.jsonpCallback;if(b&&!window[b]){window[b]=function(d){a.success.call(a,d,"success");try{delete window[b];}catch(c){window[b]=undefined;}};}};

usage.js

/**
 * The jQuery.jsonpCallback plugin enables support for the jsonpCallback Ajax setting, 
 * introduced in jQuery 1.4, to earlier versions of jQuery. This property is useful in
 * situations where dynamic naming of a JSON-P callback is not possible, ie. environments
 * with cached services.
 *
 * This plugin is backward- and forward-compatible.
 */

/**
 * Detecting support
 */
if (jQuery.support.jsonpCallback) {
  // jsonpCallback is natively supported
} else {
  // jsonpCallback is NOT natively supported
}


/**
 * Basic usage
 */
$.ajax({
  url: 'test.js',
  dataType: 'jsonp',
  
  // Enable jsonpCallback support
  beforeSend: $.useJsonpCallback,

  // Name fixed jsonpCallback method
  jsonpCallback: 'jsonp_callback_method_name',
  
  success: handleSuccess
});


/**
 * Usage with a custom beforeSend method
 */
$.ajax({
  url: 'test.js',
  dataType: 'jsonp',
  
  beforeSend: function () {

    // Enable jsonpCallback support
    $.useJsonpCallback.apply(this, arguments);

    // do something...
  },

  // Name fixed jsonpCallback method
  jsonpCallback: 'jsonp_callback_method_name',

  success: handleSuccess
});