var hv = $('hidClientField').val();`

<asp:HiddenField ID="hidClientName" runat="server" />`

hidClientName.Value = Request.QueryString["Client_Name"] ?? "";`

var hv = $("#"+ '<%= hidClientField.ClientID %>').val();

// 'id$' will cause jQuery to search control whose ID ends with 'hidClientField'
var hv = $('input[id$=hidClientField]').val();

var hv = $('#hidClientField').val();

var hv = $('input[id$=hidClientField]').val();

var hv = $('#hidClientName').val();

var hv = $('#<%=hidClientName.ClientID%>').val();

<asp:HiddenField ID="HiddenFieldServerDateTime"
                 runat="server"
                 CssClass="SomeStyle" />

var serverDateTime = $("#<%= HiddenFieldServerDateTime.ClientID %>").val();

<div class="date-time-wrap">
  <asp:HiddenField ID="..." runat="server" />
</div>

var serverDateTime = $('.date-time-wrap input[type=hidden]').val();

var hval = $('#<%= hdMarkupPercentage.ClientID%>').val();

var hv = $("#<%= hidClientField.ClientID %>").val();