Storing javascript variables into c# variables

I have this code in javascript:
var x = e.mapX;
It gets the X-coordinate of a map. What I want to do is that I want to store this into a c# variable. I have a class named Test with an integer property X. I want to store var x into X. In the codebehind, I have this on the Page_Load:
Test test = new Test();
Then I am trying this on the javascript code:

var x = e.mapX;               //var a = '<%= test.X %>';               //document.getElementById(a).value=x;&nbs ...