How to run a javascript before postback which is called on window load

Hello all,

  I am working on a web application. In one of my content pages, I wanted to check the internet connection before every postback. If there is no inernet connection then the postback should not occur.

This is my function to check the internet connection.

function checkinternet()
{

 $.ajax({
  url: 'http://xxx.xxx.xx.xx/driver',
  success: function(data) {
   alert('Connection.');
  },
  error: function(data) {
    alert('No Internet Connection!');
    return false;
  }
});
}

I have a setInterval in javascript which runs another javascript function(check the internet connection and then to get the GEOCODES) every 30 seconds

  function initiate_geolocation() {  
 
         // check the internet connection
         checkinternet()
         
         navigator.geolocation.getCurrentPosition(handle_geolocation_query);  
         return false;
         }

I called the function on window load..like this..

          var OnLoad = 'initiate_geolocation()';
          window.onload = function() {eval(OnLoad);};


         window.load = setInterval(function() {eval(OnLoad);}, 30000);

My question is ,


1) I reallly want the javascript to be executed first and then the code behind based on the result in the javascript.

    I know that if it is a button then i can do onclientClick.. but I am calling this javascript on window load, How do i do this?

     I really do not want to display a error page  if there is no internet connection. I want to be in the same screen where the driver was when he lost the internet connection..

Appreciate the help

Thanks