IO monad in JS

io.js

const IO = effect =>
  ({
    effect,
    run() {
      return this.effect()
    },
    map(fn) {
      if (!fn ?? typeof fn !== 'function') {
        throw new Error('.map requires a function argument.')
      }
      return IO(() => fn(this.effect()))
    },
    chain(fn) {
      if (!fn ?? typeof fn !== 'function') {
        throw new Error('.chain requires a function argument.')
      }
      return IO(() => fn(this.effect())).run()
    },
    init() {
      if (!effect ?? typeof effect !== 'function') {
        throw new Error('IO requires a function argument.')
      }
      return this
    }
  }.init())

const ioValue = value => IO(() => value)
const ioLog = message => IO(() => console.log(message))
const obj = {answer: 42}

const read = object => key => IO(() => object[key])
const write = object => key => value => IO(() => (object[key] = value))

read(obj)('answer')
  .map(String.fromCharCode)
  .chain(
    x =>
      console.log(
        `The answer to everything is ${x}.`
      ) // The answer to everything is *.
  )

write(obj)('answer')(0).run() // 0

ioValue(20)
  .map(x => x * 2)
  .chain(x => ioValue(1).map(y => x + y * 2))
  .chain(ioLog)
  .run() // 42