Как работает сериализация?

var room = { 
  number: 23, 
  occupy: function() { 
    alert( this.number ); 
  } 
}; 
 
var event = { 
  title: "Конференция", 
  date: new Date(Date.UTC(2014, 0, 1)), 
  room: room 
}; 
 
 
console.log(JSON.stringify(event))

var room = { 
  number: 23, 
  toJSON: function() { 
    console.log( this.number ); 
  } 
}; 
 
var event = { 
  title: "Конференция", 
  date: new Date(Date.UTC(2014, 0, 1)), 
  room: room 
}; 
 
 
console.log(JSON.stringify(event))