Question

Is there any way to change the value of a code behind property from JavaScript without using hidden-fields?

Answer 1

No it could not be done without sending request to server either by postback or ajax etc. This could help you sending request to server using jquery Ajax

If you want to change the value of some server control then it is possible with javascript.

In html

<asp:TextBox id="txtName" runat="server" ></asp:TextBox>

In javascript

<script type="text/javascript>
     txtName = document.getElementById('<%= txtName.ClientId %>');
     txtName.value = "changed";
     alert(txtName.value);         
</script>