$.requires function for loading external javascript requirements

jquery.requires.js

window.requires_urls = {};

(function($){

$.requires = function(url, callback) {
  // look for the url in our cached url results
  for(called_url in window.requires_urls) {
    var cached = window.requires_urls[called_url].cached;
    // if a url matches and it has cached results
    if(url_equals(url, called_url) && cached.data) {
      // run the callback, if any
      if(callback) callback(cached.data, cached.textStatus);
      // return the xmlHttpRequest, and we're done
      return cached.data.xmlRequest;
    }
  }

  // this will be the xmlHttpRequest from the first time we called this url
  var xmlRequest;

  // if we haven't heard of this url before
  if(!window.requires_urls[url])
  {
    // build it's data structure
    window.requires_urls[url] = {
      callbacks : [],
      cached : {}
    };
   
    // push this callback into the queue of callbacks
    window.requires_urls[url].callbacks.push(callback);

    // do the ajax request, storing the xmlHttpRequest
    xmlRequest = $.ajax({
      url: url,
      type: 'get',
      cache: true,
      dataType: 'script',
      global: false,
      success: function(data, textStatus) {
        // after the request finishes do:
        // for each callback in the queue
        var callbacks = window.requires_urls[url].callbacks;
        for (var i in callbacks) {
          var this_function = callbacks[i];
          // run it if it a function
          if(typeof this_function == 'function') this_function(data, textStatus);
        }
        // empty the queue
        window.requires_urls[url].callbacks = [];
        // cache the results of the ajax
        window.requires_urls[url].cached = {data: data, textStatus: textStatus, xmlRequest: xmlRequest};
    }});

    // store the xmlHttpRequest
    window.requires_urls[url].cached.xmlRequest = xmlRequest;
  } else {
    // we have heard of this url before so:
    // grab the xmlHttpRequest from the first call
    xmlRequest = window.requires_urls[url].cached.xmlRequest;
    // and push our callback into our queue
    window.requires_urls[url].callbacks.push(callback);
  }

  // return the xmlHttpRequest
  return xmlRequest;
};

})(jQuery);