maybe-monad.js

const Just = value => ({
  map: f => Just(f(value)),
  bind(f) { return this.map(f).join() },
  join: () => value,
  isJust: () => true,
  isNothing: () => false,
  orJust: _ => value
})

const Nothing = () => ({
  map: f => Nothing(),
  bind: f => Nothing(),
  join: () => Nothing(),
  isJust: () => false,
  isNothing: () => true,
  orJust: value => value
})

// Maybe[Nothing,Just]
const Maybe = val => {
  if (!val) return Nothing()
  
  return Just(val)
}
Maybe.of = val => Maybe(val)