Why isn't this recursion working on Javascript?

Question

The problem is in the listToArray function, arrayToList just there to provide context on how lists are being created.

var arrayToList = function (array) {
    var lastList = null;
    for (i=array.length-1; i >= 0; i--) {
        var list = {value : array[i], rest : lastList};
        lastList = list;
    }
    return lastList;
}

var list = arrayToList([1,2,3])

var listToArray = function (list) {
    var array = [];
    array.push(list.value);
    if (list.rest != null) {
        array.concat(listToArray(list.rest));
    } else {
        return array;
    }
}

var array = listToArray(list)

> list
{ value: 1, rest: { value: 2, rest: { value: 3, rest: null } } }
> array
undefined

There’s only a return statement in the else block, not in the if block. — Sebastian Simon
– Sebastian Simon, Commented Aug 4, 2016 at 1:42
Is this really something that could not be solved by stepping through your code in the debugge? — user663031
– user663031, Commented Aug 4, 2016 at 2:38

Ayan · Accepted Answer · 2016-08-04 01:52:14Z

1

Couple of fixes:

Have to return the array to be used in array.concat
Every time a new arr = [] loses the context of the previous. Either it should be passed as a parameter, or keeping it in a global context.

Run the following snippet.

var arrayToList = function(array) {
  var lastList = null;
  for (i = array.length - 1; i >= 0; i--) {
    var list = {
      value: array[i],
      rest: lastList
    };
    lastList = list;
  }
  return lastList;
}

var list = arrayToList([1, 2, 3])

var listToArray = function(list, array) {
  if (!array) {
    array = [];
  }
  array.push(list.value);
  if (list.rest != null) {
    array.concat(listToArray(list.rest, array));
  } else {
    return array;
  }
  // need to return the array which is used in array.concat
  return array;
}
var array = listToArray(list);
console.log(list);
console.log(array);

edited Aug 4, 2016 at 1:52

answered Aug 4, 2016 at 1:45

Ayan

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