Show alert when user enter less than 11 digit number in ASP.Net using JavaScript

<input name="txtDigits" type="text" id="txtDigits" onchange="Validate(this.value)"
    onkeypress="return IsNumeric(event);" ondrop="return false;" onpaste="return false;" />
<span id="lblError" style="display: none; color: red">11 digits required.</span>
<script type="text/javascript">
    function Validate(value) {
        document.getElementById("lblError").style.display = value.length != 11 ? "block" : "none";
    }

    var specialKeys = new Array();
    specialKeys.push(8);
    function IsNumeric(e) {
        var keyCode = e.which ? e.which : e.keyCode
        var ret = ((keyCode >= 48 && keyCode <= 57) || specialKeys.indexOf(keyCode) != -1);
        return ret;
    }
</script>