The Ajax.Request Library Class

      function zipChanged(zip) {
          if(zip.length != 5) {
              clearCityAndStateFields();
          }
          else {
              new Ajax.Request(
               "zipChanged.ajax",   // URL
                 {     method: "get", // HTTP method
                   parameters: "zip=" + zip, // Request params
                   onComplete: processZipCodeSelection, // callback
                 });
          }
      }

• If the length of the zip code string is not 5, the city and state fields are cleared
• The third argument to the constructor, of the form { . . . }, is a small hash table
• The server needs the submitted zip to check, so it is sent in a request parameter
• When the response is complete, the callback function processZipCodeSelection is invoked on the client