README.md

Memento Pattern

The Memento pattern lets an object save and restore its own state without exposing that state to the outside world. It's the backbone of undo/redo: the object stays in control of what gets snapshotted and how it's restored.

Example

class Memento {
  constructor(state) {
    this.state = state;
  }

  getState() {
    return this.state;
  }
}

// Originator
class Originator {
  constructor() {
    this.state = "";
  }

  setState(state) {
    this.state = state;
    console.log(`State set to: ${this.state}`);
  }

  saveStateToMemento() {
    return new Memento(this.state);
  }

  getStateFromMemento(memento) {
    this.state = memento.getState();
    console.log(`State restored to: ${this.state}`);
  }
}

// Caretaker
class Caretaker {
  constructor() {
    this.mementoList = [];
  }

  add(memento) {
    this.mementoList.push(memento);
  }

  get(index) {
    return this.mementoList[index];
  }
}

Explaining the code

Classes and Methods

1. Memento

• Purpose: Stores the state of the Originator.
• Constructor: Takes a state parameter and assigns it to the instance.
• getState(): Returns the stored state.

2. Originator

   • Purpose: Creates and stores states in Memento objects.
   • Constructor: Initializes the state to an empty string.
   • setState(state): Sets the current state and logs it.
   • saveStateToMemento(): Creates a new Memento with the current state.
   • getStateFromMemento(memento): Restores the state from a given Memento and logs it.

3. Caretaker

   • Purpose: Manages the Memento objects.
   • Constructor: Initializes an empty list to store Memento objects.
   • add(memento): Adds a Memento to the list.
   • get(index): Retrieves a Memento from the list by index.

Usage

const originator = new Originator();
const caretaker = new Caretaker();

originator.setState("State #1");
originator.setState("State #2");
caretaker.add(originator.saveStateToMemento());

originator.setState("State #3");
caretaker.add(originator.saveStateToMemento());

originator.setState("State #4");

console.log("Current State:", originator.state);
originator.getStateFromMemento(caretaker.get(0));
console.log("First saved State:", originator.state);
originator.getStateFromMemento(caretaker.get(1));
console.log("Second saved State:", originator.state);

• Create instances of Originator and Caretaker.
• Set various states in the Originator.
• Save states to Memento objects and store them in the Caretaker.
• Restore states from Memento objects using the Caretaker.

Summary

The three-role split keeps things clean: Originator knows its own state, Memento holds a snapshot of it, and Caretaker manages the list of snapshots. The caretaker never peeks inside a memento; it just stores and retrieves them. That's what makes this encapsulation-friendly compared to other undo approaches.