Passing Current User to a Jquery

Is someone able to take this question. I don't want to repost the question. Thanks for your help.

I need to pass the current user to a jquery from a Document Library. I have passed the current user before, but I had the use of a Transformation where I could set it and then pass it. The Document Library has no transformation, so I am hoping someone knows how to pass the current user along to a script.

Thanks.

This is my jquery

$(document).on("click", "a", function() 


                  $.ajax({
                   type: "POST",
                  url: "/CMSPages/WebService.asmx/WhoClicked",
                   async: false,
              //    Need to pass the username or userid here.  
                   data: JSON.stringify({ UserName: 'RT-msmith', DocumentID: myString2}),
                   // data:{}
                   contentType:  "application/json",
                   success: function (data) {  
                   alert("success");
                       },
                   error: function (XMLHttpRequest, textStatus, errorThrown) {
                     // alert(errorThrown); 
                   }


                });
});