EJ: C4 P3 created by oliverjam - https://repl.it/ESNK/1

EJ: C4 P3.js

function arrayToList(array) {
  let list = null;
  for (let i = array.length - 1; i >= 0; i--) {
    list = { value: array[i], rest: list };
  }
  return list;
}

function listToArray(list) {
  let result = [];
  for (let node = list; node; node = node.rest) {
    result.push(node.value);
  }
  return result;
}

function prepend(val, list) {
	list = { value: val, rest: list };
	return list;
}

function nth(list, index) {
	for (let node = list; node; node.rest)	{
		index -= 1;
		if (index === 0) return node.value;
	}
}

console.log(arrayToList([10, 20, 30, 40, 50]));
console.log(listToArray(arrayToList([10, 20, 30])));
console.log(prepend(10, prepend(20, null)));
console.log(nth(arrayToList([10, 20, 30]), 1));