Assigning a variable in Javascript

I have defined index as <%var index = 0;%> This is at the top of the body of HTML so I can use it throughout as a global variable. and I want to assign value of $(this).val() to <% index %>. I have tried multiple times but am not able to figure it out. I am using Node.js in backend and EJS in front end.

<script>
    $("#agency").on("change", function () {
        var $modal = $('#myModal');
            if($(this).val() !== '0'){
                <% index %> = parseInt($(this).val()); ????
            }
            $modal.modal('show');
        }
    });
</script>

via Jay N Patel