// A LIST

const arrayToList = (arr) => {
  if (arr.length == 0) return;
  let val = arr.pop();
  return { value: val, rest: arrayToList(arr) }
}

const listToArray = ({value, rest}, result = []) => {
  result.push(value);
  if (rest == null) return;
  listToArray(rest, result);
  return result;
}

const prepend = (value, rest) => ({ value, rest });

const nth = ({value, rest}, position) => {
  if (position == 0) return value;
  return nth(rest, position - 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

// DEEP COMPARISON

const deepEqual = (a, b) => {
  if (a === b) return true;

  if (a == null || typeof a != "object" ||
      b == null || typeof b != "object") return false;

  const keysA = Object.keys(a), keysB = Object.keys(b);

  if (keysA.length != keysB.length) return false;

  for (let key of keysA) {
    if (!keysB.includes(key) || !deepEqual(a[key], b[key])) return false;
  }

  return true;
}

let obj = {here: {is: "an"}, object: 2};
console.log(deepEqual(obj, obj));
// → true
console.log(deepEqual(obj, {here: 1, object: 2}));
// → false
console.log(deepEqual(obj, {here: {is: "an"}, object: 2}));
// → true