title: extJs Json Reader
body: <p>Please help me with this problem. I'm new in extJs and i need a little help.
  I have this code</p>
  
  <pre><code>Ext.onReady(function() {
   var datesStore = new Ext.data.JsonStore({
    start : 'StartTableDate',
    end : 'FinishTableDate',  
              autoLoad : true,
              proxy : new Ext.data.HttpProxy({
                  url : 'dates.json',
                  method:'GET'
              }),
              fields : [
                  // 2 mandatory fields
                  {name:'StartTableDate'},
                  {name:'FinishTableDate'}
              ]
          });
  
  // i want to pass to variable start si end the values from JSON
  var start = 'StartTableDate';
  var end = 'FinishTableDate';
  </code></pre>