Refactoring of http://sharepoint.stackexchange.com/q/53484/627

SPGetSPcurrentUser.js

var SPGetSPcurrentUser = function(callback) {
    var spuserid = "Error";
    var context = new SP.ClientContext.get_current();
    // Using this here would make it global, as the function is not called with 'new'
    // and this would then point to the global object, window
    //this.website = context.get_web();
    var website = context.get_web();
    var currentUser = website.get_currentUser();
    context.load(currentUser);
    context.executeQueryAsync(

    function(sender, args) {
        callback(currentUser.get_loginName());
    }, function(sender, args) {
        this.spuserid = 'request failed ' + args.get_message() + '\n' + args.get_stackTrace();
    });
};

var thisuser = SPGetSPcurrentUser(function(thisuser) {
    alert(thisuser);
});