Excises of Eloquent JavaScript 4th edition. Chapter 04 Problem 03. https://eloquentjavascript.net/04_data.html#i_6xTmjj4Rf5

Eloquent JavaScript 0403 A list.js

// Your code here.
const arrayToList = (arr) => {
  return arrayToListR(arr, 0);
}

const arrayToListR = (arr, start) => {
  if (start >= arr.length) return null;
  
  const ans = {};
  ans.value = arr[start];
  ans.rest = arrayToListR(arr, start + 1);
  
  return ans;
}

const listToArray = (list) => {
  const ans = [];
  let curr = list;
  
  while (curr) {
    ans.push(curr.value);
    curr = curr.rest;
  }
  
  return ans;
}

const prepend = (value, list) => {
  const head = {
    value,
    rest: list
  }
  
  return head;
}

const nth = (list, index) => {
  if (index == 0) return list.value;
  return nth(list.rest, index - 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