memoized_y_combinator.js

const mkWrapper = () => (
  cache => f => n => n in cache
    ? cache[n]
    : cache[n] = f(n)
)([]);
const M = f => f(f);
const Y = f => (
  wrapper => M(
    maker => n => wrapper(
      f(M(maker))
    )(n)
  )
)(mkWrapper());

const fact = Y(self => n => (
  console.log(n),
  n <= 0
    ? 1
    : n * self(n-1)
  )
);

console.log(fact(6));
console.log(fact(7));