// Y combinator
// \f.(\x.f (x x)) (\x.f (x x))
// Translated from https://gist.github.com/logicmason/0722b5b159a45f7a81b6

var Y = fn => 
  (x => fn(y => x(x)(y)))
  (x => fn(y => x(x)(y)))
;

var factgen = (fact) =>
  n => (n === 0) ? 1 : n * fact(n-1)
;

var factorial = Y(factgen);
console.log(factorial(5));