JS Promise and Applicative Functor

index.js

Promise.prototype.ap = function(p) {
  const self = this;
  return p.then(v => self.then(f => f(v)));
}

const add =
  a => b => a + b;

console.log(add(1)(2)); // 3

Promise.resolve(add)
  .ap(Promise.resolve(1))
  .ap(Promise.resolve(2))
  .then(console.log); // 3


Promise.resolve(1)
  .then(add)
  .ap(Promise.resolve(2))
  .then(console.log); // 3

Promise.resolve(add(1))
  .ap(Promise.resolve(2))
  .then(console.log) // 3