Problem assigning string to .AJAX URL property

When attempting to set the url to a constructed value using an anonymous function:

$.ajax({url:function () {
                    var url = ... // construct URL here
                    return url;
                  }

I discovered that the url became set to a string which contained the function definition,

rather than the string value which is returned ''from that function''.

This appears to me to be a bug.

There is a workaround, using javascript's IIFE construct thus:

$.ajax({url: (function () {
                    var url = ... // construct URL here
                    return url;
                  }
              )() // using IIFE