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 array = [];

for (let node = list; node; node = node.rest) {

array.push(node.value);

}

return array;

}

function prepend(value, list) {

return {value, rest: list};

}

function nth(list, n) {

if (!list) return undefined;

else if (n == 0) return list.value;

else return nth(list.rest, n - 1);

}

console.log(arrayToList([10, 20]));

// → {value: 10, rest: {value: 20, rest: null}}

console.log(listToArray(arrayToList([10, 20, 30])));

// → [10, 20, 30]

console.log(prepend(10, prepend(20, null)));

// → {value: 10, rest: {value: 20, rest: null}}

console.log(nth(arrayToList([10, 20, 30]), 1));

// → 20