JS data structures - Doubly Linked List

doubleLinkedList.js

'use strict';

function Node(v) {
  this.value = v;
  this.next = null;
  this.prev = null;
}

function DoubleLinkedList() {
  this.length = 0;
  this.head = null;
  this.tail = null;
}

DoubleLinkedList.prototype.append = function (item) {
  var node = new Node(item);
  if (this.tail === null) {
    this.head = node;
    this.tail = node;
  } else {
    this.tail.next = node;
    node.prev = this.tail;
    this.tail = node;
  }

  this.length++;
};

DoubleLinkedList.prototype.prepend = function (item) {
  var node = new Node(item);
  if (this.head === null) {
    this.head = node;
    this.tail = node;
  } else {
    this.head.prev = node;
    node.next = this.head;
    this.head = node;
  }

  this.length++;
};

DoubleLinkedList.prototype.length = function () {
  return this.length;
};

DoubleLinkedList.prototype.remove = function (idx) {
  if (idx >= this.length) {
    throw new Error('out of range');
  }

  // Remove first element
  if (idx === 0) {
    this.head = this.head.next;
    this.head.prev = null;
    // Remove final element
  } else if (idx === this.length) {
      this.tail = this.tail.prev;
      this.tail.next = null;
      // Remove any element inside
    } else {
        var count = 0;
        var curr = this.head;
        var prev = null;
        while (count < idx) {
          count++;
          prev = curr;
          curr = curr.next;
        }
        prev.next = curr.next;
      }

  this.length--;
};

DoubleLinkedList.prototype.item = function (idx) {
  if (idx >= this.length) {
    throw new Error('out of range');
  }
  var count, curr;
  // Item in first half; iterate from head of list
  if (idx < this.length / 2) {
    count = 0;
    curr = this.head;
    while (count < idx) {
      count++;
      curr = curr.next;
    }
    return curr.value;
    // Item in last half; iterate from end of list
  } else {
      count = this.length - idx;
      curr = this.tail;
      while (count >= idx) {
        count--;
        curr = curr.prev;
      }
      return curr.value;
    }
};