My first attempt to understand Monads in JavaScript

monads.js

// Heavily based on this great article by Eric Elliott:
// https://medium.com/javascript-scene/javascript-monads-made-simple-7856be57bfe8

const main1 = () => {
  const compose = (f, g) => (x) => f(g(x));

  // g :: Integer -> [Integer]
  const g = (a) => Array.of(a);

  // f :: [Integer] -> Bool
  const f = (b) => b.reduce((x, y) => x + y) > 0;

  // h :: Integer -> Bool
  const h = (x) => compose(f, g)(x);

  const result = h(3); // f(g(3)) -> Array.of(3) -> [3].reduce((x, y) => x + y) > 0
  console.log(result); // true
};

const main2 = () => {
  // Identity monad
  const Id = value => ({
    map: f => Id.of(f(value)),
    chain: f => f(value),
    toString: () => `Id(${value})`
  });
  Id.of = Id;

  const g = n => Id(n + 1);
  const f = n => Id(n * 2);

  const result = Id(20).chain(f).chain(g).toString(); // 20 * 2 -> 40 + 1
  console.log(result); // Id(41)
};

main1();
main2();