Nodejs: Do singleton variables get overwritten do to async requests?

var Singleton = {
    Sdata : "some data",

    modify-data : function(data, callback){
            Sdata = data;
            callback(Sdata);
    }
}

Here, does Sdata get overwritten by two different requests accessing the function modify-data at the same time?

Can it be averted like following:

var Singleton = {

    modify-data : function(data, callback){
            var Sdata = data;
            callback(Sdata);
    }
}

here, does a new var Sdata get created every time? And if two requests access modify-data at the same time, does it create a new instance of Sdata for each?