Sharepoint - How to return a value using context.executeQueryAsync?

As the method executeQueryAsync indicates it is asynchronous.
You can actually force the XMLHttpRequest to be synchronous, although that is beside the point.

What you are looking for is a callback function, something like this:

var getSpCurrentUser = function(callback) {
  var ctx = SP.ClientContext.get_current();
  var user = ctx.get_web().get_currentUser();

  ctx.load(user);
  ctx.executeQueryAsync(function() {
    callback(null, user);
  }, function(a, b) {
    callback(new Error(b.get_message()));
  });
};

getSpCurrentUser(function(error, user) {
  if (error) {
    console.error(error);
    return;
  }
  console.log(user.get_title());
});