socket.io socket.set and socket.get - what is the callback argument for?

Question

I just want to attach some variables to my sockets, but I don't understand the .get and .set methods. They both have a third argument for a callback. Can this be omitted? What is it for?! Does the callback itself intherit arguments?

My best guess is that it's there for when socket.io isn't using memory store but is saving this stuff somewhere else and needs an error checking callback.

socket.on('set nickname', function (name) {
    socket.set('nickname', name, function (arguments) {
      doSomethingMysterious();
    });

I'm using socket.io 0.9.x

The function is defined in socket.js

/**
 * Stores data for the client.
 *
 * @api public
 */

Socket.prototype.set = function (key, value, fn) {
  this.store.set(key, value, fn);
  return this;
};

Answer 1

The get and set functions on the socket object were removed in version 1.x. The proper way to store and retrieve values now is through properties on the socket object, like so:

socket.on('set nickname', function (name) {
  socket.nickname = name;
});

As seen on the example code from the official repo.