Write a function arrayToList that builds up a data structure like the previous one when given [1, 2, 3] as argument, and write a listToArray function that produces an array from a list. Also write the helper functions prepend, which takes an element and a list and creates a new list that adds the element to the front of the input list, and nth, …

4-3-alist.js

/*
http://eloquentjavascript.net/04_data.html

Write a function arrayToList that builds up a data structure like the previous one when given [1, 2, 3] as argument, and write a listToArray function that produces an array from a list. Also write the helper functions prepend, which takes an element and a list and creates a new list that adds the element to the front of the input list, and nth, which takes a list and a number and returns the element at the given position in the list, or undefined when there is no such element.

If you haven’t already, also write a recursive version of nth.
*/

function arrayToList( array ) {
  var list = null;

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

  return list;
}

function listToArray( list ) {
  var array = [];

  for( var node = list; node; node = node.rest )
    array.push( node.value );

  return array;
}

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

function nth( list, n ) {
  // Simpler solution
  //
  // return listToArray( list )[n];
  
  // Recursive solution
  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