MyMaybe.js

import { pipe, propEq } from 'ramda'
import { prop } from 'crocks'

class MyMaybe {
  #value = null
  #isNothing = true

  constructor(value) {
    if (!value) 
      this.#isNothing = true
    else {
      this.#isNothing = false
      this.#value = value
    }
  }

  static Just(value) {
    return new MyMaybe(value)
  }

  static Nothing() {
    return new MyMaybe()
  }

  map(f) {
    return this.#isNothing ? 
      MyMaybe.Nothing() : 
      MyMaybe.Just(f(this.#value))
  }

  chain(f) {
    return this.#isNothing ?
      MyMaybe.Nothing() : 
      f(this.#value)
  }

  get(f) {
    this.#isNothing ? 
      f('Nothing') :
      f(this.#value)
  }
}

let find = f => a => {  
  let result = a.find(f)
  return (result === undefined) ? MyMaybe.Nothing() : MyMaybe.Just(result)
}

let data = [
  { title: 'FP in JavaScript', price: 100 },
  { title: 'RxJS in Action', price: 200 },
  { title: 'Speaking JavaScript', price: 300 }
]

let result = find(x => x.title === 'Speaking JavaScript')(data)
result.get(x => console.log(x))

let result_ = find(x => x.title === 'RxJS')(data)
result_.get(x => console.log(x))