How to call a webservice method from an html page [javascript] with out refresh the page

Use jQuery for performin POST or GET request from your html page like this :

function fun() 
{
   var data="hello";
   $.get("http://localhost/ws/service.asmx/HelloWord", function(response) {
        data = response;
   }).error(function(){
  alert("Sorry could not proceed");
});

   return data;
}

OR :

function fun() 
{
  var data="hello";
  $.post('http://localhost/ws/service.asmx/HelloWord',{},function(response) 
  {     data = response;
  }).error(function(){
  alert("Sorry could not proceed");
});

    return data;
}

You can send ajax request to a webservice

 $.ajax({
            url: "WebServiceURL",
            data: "", //ur data to be sent to server
            contentType: "application/json; charset=utf-8", 
            type: "GET",
            success: function (data) {
               alert(data);
            },
            error: function (x, y, z) {
               alert(x.responseText +"  " +x.status);
            }
        });