y-combinator

y-combinator.js

// basic idea
(f => f(f)(5))(f => n => n == 0 ? 1 : (n * f(f)(n-1)));

// This version doesn't work, because js is not lazy.
// const Y = f => (x => f(x(x)))(x => f(x(x)));

// const Y = f => (x => x(x))(x => f(a => x(x)(a)));

const Y = f => (x => f(y => x(x)(y)))(x => f(y => x(x)(y)));

Y(fact => n => n == 0 ? 1 : n*(fact(n-1)))(5);