4.2

// Your code here.
function arrayToList(arr){
  var list = null;
  for (var i = arr.length-1; i>=0; i--)
     list = {value:arr[i], rest:list};
  return list;
}

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

function prepend(value, list) {
  return {value: 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