mapReduce.js

function mapReduce(map, reduce, zero){
	return function loop(from, to){
		if(from > to){
			return zero;
		}

		return reduce(map(from), loop(from + 1, to));
	};
}

function mult(a, b){
	return a * b;
}

function id(a){
	return a;
}

function fact(n){
	return mapReduce(id, mult, 1)(1, n);
}

fact(5);

function gcd(a, b){
	return b === 0 ? a : gcd(a, b % a);
}

gcd(2, 4)