Example - Singleton DP in Node.js

singleton-example.js

/**
 * This is a mocked connection class. Think of it like a class to create a connection to database.
 */
class Connection {
  status = '';

  constructor() {
    console.log('instantiated');
    this.status = 'connected';
    console.log(this.status);
  }
}

/**
 * SingletonConnection 
 * We can apply Singleton DP with the Connection class,
 * and instantiate x and y to see status changed in x is also changed in y.
 */
class SingletonConnection {
  static _instance;
  static getInstance() {
    if (!SingletonConnection._instance) {
      SingletonConnection._instance = new Connection();
    }

    return SingletonConnection._instance;
  }
}

// x instantiates a connection
const x = SingletonConnection.getInstance();

// y is created later, Singleton design pattern will use the same instance previously instantiated
const y = SingletonConnection.getInstance();

// So when x is closed
x.status = 'closed';
console.log('x.status', x.status);

// y also closed (as it refers to the same instance)
console.log('y.status', y.status);