composite.js

function canEat(obj) {
  obj.eat = eat;
  obj.energy = 0;
  
  function eat(energy) {
    if (!this) { return; }

    this.energy = this.energy || 0;

    this.energy += energy;

    return this.energy;
  }
}

class Rabbit {
  constructor(name) {
    this.name = name;
  }
}

let phillip = new Rabbit('phillip');

console.log(phillip.name); // 'phillip'
console.log(phillip.eat); // undefined

phillip.eat = eat;

console.log(phillip.eat) // undefined