reduce-implementation.js

const someArray = [4,3,3,2,5,6,63,1,43,34,46,46,2,35,35,64,75];

function reduction(list, callback) {
	if (typeof callback !== 'function') {
		throw new Error('callback not function');
	}
	let count = 1;

	function reducting(current) {
		if (++count > list.length - 1) {
			return current;
		}

		return reducting(callback(current, list[count], count, list));
	}

	return reducting(callback(list[0], list[1]));
}

const result = reduction(someArray, (x, y) => x + y);

console.log(result);