README.md

Memento Pattern

The Memento pattern is used to capture and externalize an object's internal state so that it can be restored later without violating encapsulation. This is useful for implementing features like undo/redo functionality.

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 Originator class can save its state to a Memento object and restore it later. The Caretaker class manages these Memento objects, allowing the Originator to revert to previous states. The Memento pattern is useful for scenarios where you need to implement undo/redo functionality.