Ajax send function in data property [duplicate]

I need some reactivity in vanila js. I solved the problem but ran into a problem. Simple example:

Why does it work correctly:

       let data = {
           'test': ()=>"some value"
       }

        $.ajax({
            url: "/api/test",
            dataType: "json",
            type: 'post',
            data: data,
            success: function(data){
            }});

and it doesn’t work:

        let data = {}
        Object.defineProperty(data,'test', {
            get: () => "some test value"
        }
        $.ajax({
            url: "/api/test",
            dataType: "json",
            type: 'post',
            data: data,
            success: function(data){
            }});
    }