Javascript call Server-side function thru XMLHttpRequest

xmlHttp = new ActiveXObject("Msxml2.XMLHTTP");
xmlHttp.open("POST", "custom.aspx", true);       

xmlHttp.send();

xmlHttp.onreadystatechange = function() {

            if (xmlHttp.readyState == 4) {}
}


the above is the code used to invoke a page thru XmlHttpRequest object. The code above calls the custom.aspx's page_load method, if , however I want to call a method CalculateAmount() of custom.aspx in the same page, how can i achieve it.
Note: Not by using any of the frameworks.



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