Singleton Pattern

class-singleton.js

/**
 * The singleton pattern is a pattern used to restrict an object to a single instance. 
 * A singleton creates an instance of an object if that object does not exist. 
 * If it exists it returns the existing instance.
 */

class Car { 
  constructor(model, year, miles) { 
    if(Car.exists){ 
      return Car.instance 
    } 
      this.model = model; 
      this.year = year; 
     this.miles = miles; 
      Car.exists = true; 
      Car.instance = this; 
      return this 
    }

    toString(){ 
      return this.model + " has done " + this.miles + " miles"; 
    } 
} 

// Usage: 
    
let civic = new Car( "Honda Civic", 2009, 20000 ); 
let mondeo = new Car( "Ford Mondeo", 2010, 5000 ); 
    
console.log( civic.toString() ); // Honda Civic has done 20000 miles 
console.log( mondeo.toString() ); // Honda Civic has done 20000 miles