Memento

Capture and restore an object's internal state

• The Memento pattern provides temporary storage as well as restoration of an object.

• The mechanism in which you store the object’s state depends on the required duration of persistence, which may vary.

• You could view a database as an implementation of the Memento design pattern in which objects are persisted and restored.

• However, the most common reason for using this pattern is to capture a snapshot of an object’s state so that any subsequent changes can be undone easily if necessary.

• In JavaScript Mementos are easily implemented by serializing and de-serializing objects with JSON.

EX:
var Person = function(name, street, city, state) {
    this.name = name;
    this.street = street;
    this.city = city;
    this.state = state;
}
 
Person.prototype = { 
    hydrate: function() {
        var memento = JSON.stringify(this);
        return memento;
    }, 
    dehydrate: function(memento) {
        var m = JSON.parse(memento);
        this.name = m.name;
        this.street = m.street;
        this.city = m.city;
        this.state = m.state;
    }
}
 
var CareTaker = function() {
    this.mementos = {}; 
    this.add = function(key, memento) {
        this.mementos[key] = memento;
    }, 
    this.get = function(key) {
        return this.mementos[key];
    }
} 
 
function run() {
    var mike = new Person("Mike Foley", "1112 Main", "Dallas", "TX");
    var john = new Person("John Wang", "48th Street", "San Jose", "CA");
    var caretaker = new CareTaker();
 
    // save state 
    caretaker.add(1, mike.hydrate());
    caretaker.add(2, john.hydrate());
 
    // mess up their names 
    mike.name = "King Kong";
    john.name = "Superman";
 
    // restore original state 
    mike.dehydrate(caretaker.get(1));
    john.dehydrate(caretaker.get(2));
    console.log(mike.name);
    console.log(john.name);
}

run();