UserId availability check textbox is not working

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C#

ASP.NET

I am using a input button like this:
<input id="btnDoAjaxCall" type="button" value="Check Availability" onclick="return btnDoAjaxCall_onclick()" />
but when i click this button it does not work.

i also used master page in the project.
this is my javascript code:

<script language="javascript" type="text/javascript">
    function btnDoAjaxCall_onclick() {
        var xmlhttp;
        if (window.XMLHttpRequest) {
            xmlhttp = new XMLHttpRequest();
        }
        else {
            xmlhttp = new ActiveXObject("Microsoft.XMLHTTP");
        }
        xmlhttp.onreadystatechange = function () {
            if (xmlhttp.readyState == 4 && xmlhttp.status == 200) {
                document.getElementById("showAvailability").innerHTML = xmlhttp.responseText;
            }
        }
        var str = document.getElementById("txtEmail").value;
        //sending request to handler page
        xmlhttp.open("GET", "Handler.ashx?uname=" + str, true);
        xmlhttp.send();
    }
    </script>

I write this code in head of the master page. i also check this code to placed in the
<asp:Content ID="Content1" ContentPlaceHolderID="head" Runat="Server">

but it does not work. Please help how can i solve this.
This code is working in my previous project where i not used master page. But after using master page it does not work. Please help me for do this.
Thanks in advance


can any one tell me where i can write this javascript code. txtEmail textbox and showAvailability div is my another page.