getDecimal.js

const getDecimalValue = (head, [currentNode, string] = [head, '']) => {
  do {
    string += currentNode.val;
    currentNode = currentNode.next;
  } while (currentNode)
  return parseInt(string, 2);
}

const getDecimalValue = (h, [c, a] = [h, []]) => {
  do {
    a.unshift(c.val);
    c = c.next;
  } while (c)

  return a.reduce((d, n, i) => d += n * (!!n ? 2 ** i : 0), 0)
}