testES6.js

class Car {
  constructor(make, wheels) { 
    this.make = make;
    this.wheels = wheels;
    this.currentSpeed = 25;
  }

  printCurrentSpeed() {
    console.log(this.make + ' is going ' + this.currentSpeed + ' mph.');
  }

  wheelsCheckUp() {
    this.wheels.map(x => x.currentQuality());  
  }
}

class RaceCar extends Car {
  constructor(make, wheels) {
    super(make, wheels);
  }
}

class Wheel {
  constructor(quality) {
    this.quality = quality;
  }
  
  currentQuality() {
    console.log('Wheel quality: ', this.quality);
  }
}

const car = new RaceCar('BabelCar', [new Wheel('poor'), new Wheel('good'), new Wheel('excellent'), new Wheel('good enough')]);
car.printCurrentSpeed();
car.wheelsCheckUp();

// car = 1; // <- const ! read-only