const ifElse = f => g => h => x => f(x) ? g(x) : h(x);
const isZero = a => a < 1;
const mult = a => b => a * b;
const dec = a => a - 1;

const factorial = Y(B(ifElse(isZero)(K(1)))(B(S(mult))(C(B)(dec))));

factorial(4);