Applicative Maybe example in JavaScript

applicatives.js

const Nothing = { kind: "Nothing" };
const Just = (val) => ({ kind: "Just", val: () => val });

function pure(val) {
  return Just(val);
}

function fmap(f, x) {
  return (x.kind == "Nothing")
    ? Nothing
    : Just(f(x.val()))
}

function ap(f) {
  return (f.kind == "Nothing")
    ? (_) => Nothing
    : (x) => fmap(f.val(), x);
}

function main(){
  const effectA = Just(10);
  const effectB = Just(30);
  const effectC = Just(33);
  const effectD = Just(33);
  
  const collect = (a) => (b) => (c) => (d) => [a, b, c, d];

  const f = pure(collect);

  const res1 = ap (f)    (effectA);
  const res2 = ap (res1) (effectB);
  const res3 = ap (res2) (effectC);
  const res4 = ap (res3) (effectD);
  
  console.log(res4.val());
}

main()