Simple Long Polling Example with JavaScript and jQuery by Tian Davis (@tiandavis) from Techoctave.com (http://techoctave.com/c7/posts/60-simple-long-polling-example-with-javascript-and-jquery)

longPolling.js

// Long Polling (Recommened Technique - Creates An Open Connection To Server ∴ Fast)
(function poll(){
    $.ajax({ url: "server", success: function(data){
        //Update your dashboard gauge
        salesGauge.setValue(data.value);
    }, dataType: "json", complete: poll, timeout: 30000 });
})();

recursivePolling.js

// The setTimeout Technique (Not Recommended - No Queues But New AJAX Request Each Time ∴ Slow)
(function poll(){
   setTimeout(function(){
      $.ajax({ url: "server", success: function(data){
        //Update your dashboard gauge
        salesGauge.setValue(data.value);
        //Setup the next poll recursively
        poll();
      }, dataType: "json"});
  }, 30000);
})();

traditionalPolling.js

// The setInterval Technique (Not Recommended - Creates Queues of Requests ∴ Can Be Slow)
setInterval(function(){
    $.ajax({ url: "server", success: function(data){
        //Update your dashboard gauge
        salesGauge.setValue(data.value);
    }, dataType: "json"});
}, 30000);