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 = new 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());
});

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());
});