jQuery Pluginization (see comment for slide order)

jquery-pluginization.js

(function($){
  
  // Store fetched long URLs here.
  var cache = {};
  
  $.longUrl = function( url, callback, options ) {
    // Override defaults with specified options.
    options = $.extend( {}, $.longUrl.options, options );
    
    var params = {};
    
    if ( cache[ url ] ) {
      // URL exists in cache, so use it.
      callback( cache[ url ] );
    
    } else {
      params[ options.param ] = url;
      
      // Fetch JSON data.
      $.getJSON( options.api_url, params, function(data){
        if ( data[ url ] ) {
          // If the data is valid, update the cache.
          cache[ url ] = data[ url ];
          
          // Call `callback` for this element + long url.
          callback( cache[ url ] );
        }
      });
    }
  };
  
  $.fn.longUrl = function( options ) {
    // Override defaults with specified options.
    options = $.extend( {}, $.longUrl.options, options );
    
    return this.each(function(){
      var elem = $(this),
        url = elem.attr( options.attr );
      
      // Fetch long URL and call `lengthen` when done.
      $.longUrl( url, function( long_url ){
        options.lengthen( elem, long_url );
      }, options );
    });
  };
  
  // Some sensible defaults.
  $.longUrl.options = {
    api_url: 'http://www.longurlplease.com/api/v1.1?callback=?',
    param: 'q',
    attr: 'href',
    lengthen: function( elem, url ) {
      // Update the element's title attribute with `url`.
      elem.attr( 'title', url );
    }
  };
  
})(jQuery);

Here are the general steps, in-order:

1. Generic non-jQuery style code
2. The basic jQuery plugin
3. AJAX request caching
4. Using a closure
5. Global optional callback
6. Optional attribute argument
7. Optional lengthen argument
8. Reworking the optional lengthen argument
9. A single, optional options argument
10. With global defaults
11. Making things more generalized
12. Making things too generalized?