dog.js

function Dog(name) {
  this.name = name
}

Dog.prototype.bark = function bark() {
  return dog_bark(this)
}

function dog_bark(dog) {
  return `${dog.name}: Woof!`
}

// Example use of dog_bark(dog)
const spike = new Dog('Spike')
const doge = new Dog('Doge')
const barks = [ spike, doge ].map(dog_bark)

In this implementation, dog_bark contains the actual implementation. Dog.prototype.bark delegates to dog_bark and exists only for convenience.