What does ULSxSy:; do in SharePoint JS files?

Question

So I was going through init.js and sp.js and found out that nearly all of the SharePoint OOB JS functions start with the following line: "ULSxSy:;"

For example:

function ctxInitItemState(ctxCur)
{ULSxSy:;
    ctxCur.TotalListItems=0;
    ctxCur.CurrentSelectedItems=0;
    ctxCur.LastSelectableRowIdx=0;
    ctxCur.StateInitDone=true;
}
function STSPageUrlValidation(url)
{ULSxSy:;
    return PageUrlValidation(url);
}

I could not find any documentation regarding why this might be. Anybody have an idea?

Answer 1

A similar question was asked on StackOverflow before, here you'll find a short answer.

This answer provides the detailed information for your question.

This line

{ULSxSy:;

is inserted into functions to allow collect diagnostic information on the client which includes the ability to send a JavaScript callstack (including function name, and javascript file) back to the server.

In your case label ULSxSy correspond to function declaration

function ULSxSy(){
   var o=new Object;
   o.ULSTeamName="Microsoft SharePoint Foundation";
   o.ULSFileName="init.debug.js";
   return o;
}