class keyword created by minmaxdata - https://repl.it/@minmaxdata/class-keyword

main.js

class Animal {
  constructor(name, energy) {
    this.name = name
    this.energy = energy
  }

  eat(amount) {
    console.log(`${this.name} is eating.`)
    this.energy += amount
  }
  sleep(length) {
    console.log(`${this.name} is sleeping.`)
    this.energy += length
  }
  play(length) {
    console.log(`${this.name} is playing.`)
    this.energy -= length
  }
}

const leo = new Animal('Leo', 7)
const snoop = new Animal('Snoop', 10)
leo.eat(10)
snoop.play(5)