Find TextBox control inside Nested DataList using JavaScript or jQuery

i have two nested datalist and there are two txtbox inside the two, now i want to write a javascript code to limit number of text insertd into but textbox, but my javascript cant find the two txtbox, see error below

Compiler Error Message: CS0103: The name 'txtcommentpost' does not exist in the current context

Source Error:


Line 4191:
Line 4192:            if (document.getElementById('<%= TextBixcomment.ClientID %>')) {
Line 4193:                 var ctrl = document.getElementById('<%= txtcommentpost.ClientID %>');
Line 4194:                 var len = document.getElementById('<%= txtcommentpost.ClientID %>').value.length;
Line 4195:                 if (len <= maxSize) {

see javascript below

 <script type="text/javascript">

        function CountCharacters() {
            var maxSize = 500;

            if (document.getElementById('<%= TextBixcomment.ClientID %>')) {
                 var ctrl = document.getElementById('<%= txtcommentpost.ClientID %>');
                 var len = document.getElementById('<%= txtcommentpost.ClientID %>').value.length;
                 if (len <= maxSize) {
                     var diff = parseInt(maxSize) - parseInt(len);

                     if (document.getElementById('<%= txtRemain2.ClientID %>')) {
                        document.getElementById('<%= txtRemain2.ClientID %>').value = diff;
                    }
                }
                else {
                    ctrl.value = ctrl.value.substring(0, maxSize);
                }
            }

            return false;
        }
    </script>

<script type="text/javascript">

    function CountCharacters() {
        var maxSize = 500;

        if (document.getElementById('<%= TextBixcomment.ClientID %>')) {
                 var ctrl = document.getElementById('<%= txtcommentpost2.ClientID %>');
                 var len = document.getElementById('<%= txtcommentpost2.ClientID %>').value.length;
                 if (len <= maxSize) {
                     var diff = parseInt(maxSize) - parseInt(len);

                     if (document.getElementById('<%= txtRemain3.ClientID %>')) {
                        document.getElementById('<%= txtRemain3.ClientID %>').value = diff;
                    }
                }
                else {
                    ctrl.value = ctrl.value.substring(0, maxSize);
                }
            }

            return false;
        }
    </script>