y-combinator.js

/*
const Y = (f) =>
  ( x => f(v => x(x)(v)) )(
    x => f(v => x(x)(v))
  );
*/

const Y = f => {
  const something = x => f(v => x(x)(v));
  return something(something);
};

const factorial = Y(
  fac => 
    n => 
      n === 0 
        ? 1 
        : n * fac(n - 1)
);

console.log(factorial(5));