storing value after Ajax function calling

Question

Hy guys i stuck in storing value to a variable after calling ajax method , i am not able to use 

callback in ajax....

 

Someone help me please...

 

 

here is the code===============

 

var s = GetSessionValue("userId");

alert(s);

 

 

function GetSessionValue(key1) {

$.ajax({

type: "POST",

url: "Service1.asmx/GetSessionValue",

data: '{key: "' + key1 + '" }',

contentType: "application/json; charset=utf-8",

dataType: "json",

success: OnSuccess,

failure: function (response) {

alert(response.d);

}

});

function OnSuccess(result) {

return result.d;

alert(result.d);

}

}

 ===============================

 

AJAX methods are calling sucessfully , data also getting correct...

 

 

but i want to return the value from ajax function and want to store in another variable .

 

 In my existing code 

var s = GetSessionValue("userId");

alert(s);

 

Im getting undefind value.

 

I know some callbak logic is required , but im not able to implement in this case ....

 

 i need someone help ...

 

Thanks