How to call a C# function from JavaScript?

You can use a Web Method and Ajax:

<script type="text/javascript">             //Default.aspx
   function DeleteKartItems() {     
         $.ajax({
         type: "POST",
         url: 'Default.aspx/DeleteItem',
         data: "",
         contentType: "application/json; charset=utf-8",
         dataType: "json",
         success: function (msg) {
             $("#divResult").html("success");
         },
         error: function (e) {
             $("#divResult").html("Something Wrong.");
         }
     });
   }
</script>

[WebMethod]                                 //Default.aspx.cs
public static void DeleteItem()
{
    //Your Logic
}

.CS File    
    namespace Csharp
    {
      public void CsharpFunction()
      {
        //Code;
      }
    }

    JS code:
    function JSFunction() {
            <%#ProjectName.Csharp.CsharpFunction()%> ;
    }

Note :in JS Function when call your CS page function.... first name of project then name of name space of CS page then function name