Problem
function Animal (name, energy) {
  let animal = {}
  animal.name = name
  animal.energy = energy

  return animal
}

Animal.prototype.eat = function (amount) {
  console.log(`${this.name} is eating.`)
  this.energy += amount
}

const leo = Animal('Leo', 7)
leo.eat(5)
Solution

Our call to leo.eat is going to fail. The reason for this is because the object returned from Animal isn’t delegating to Animal.prototype so leo.eat will be undefined.

To fix this, we can use Object.create or a combination of the this keyword with new.

function Animal (name, energy) {
  let animal = Object.create(Animal.prototype)
  animal.name = name
  animal.energy = energy

  return animal
}

// or

function Animal (name, energy) {
  this.name = name
  this.energy = energy
}

const leo = new Animal('Leo', 7)

You’ll never stop me from spreading the great this keyword.