Calling ASP.net code behind Methods from javascript

Javascript function

        function callme() {
         
            var str = '<%=GetString()%>';
            alert(str);
        }
function ClickAspbtnfromjavacript() //function for clicking asp.net button from javascript
{
 document.getElementById('btnID').click();
}
 
ASP.net (.aspx page)

body onload="callme()"

Code Behind(.cs page)


 public string GetString()
    {  
        return "Testing";
    }

Disable Previous Dates in Ajax Calendar Control in asp.net
  CalendarExtender1.StartDate = System.DateTime.Today;