OO JavaScript with super() in constructor

Animal.js

module.exports = class Animal {
  constructor(energy) {
    this.energy = energy || 0;
  }
  move() {
    if (this.energy > 0) {
      console.log('moving...')
      this.energy = this.energy - 10;
    } else {
      console.log('cannot move.. (no energy)')
    }
  }
  eat(food) {
    this.energy = this.energy + food;
  }
}

Cat.js

const Animal = require('./Animal.js');

module.exports = class Cat extends Animal {

  constructor(energy) {
    super(energy)
    this.sound = 'MEAAAAAAW!!!';
  }

  meaw() {
    console.log(this.sound)
  }
}

$ node
> Cat = require('./Cat')
> var miffy = new Cat(70);
> miffy.energy
70
> miffy.meaw()
MEAAAAAAW!!!
> miffy.move() 
moving...
> miffy.energy 
60