y-combinator.js

/**
 * The Y-Combinator
 */
const y = fn => (f => f(f))(f => fn(x => f(f)(x)));

const factorial = y(fac => n => {
  return n < 2 ? 1 : n * fac(n - 1);
});

const fibonacci = y(fib => n => {
  return n <= 2 ? 1 : fib(n - 1) + fib(n - 2);
});

console.log(factorial(5));
console.log(fibonacci(5));