function List() {
    this.length = 0;
    this._head = null;
    this._tail = null;
}

List.prototype = {

    head: function() {
      if (this.length > 0) {
          return this._head;
      } else {
          throw new Error("List has zero length.");
      }
    },

    tail: function() {
      if (this.length > 0) {
          return this._tail;
      } else {
           throw new Error("List has zero length.");
      }
    },

    append: function(value) {
        var node = {
            value: value,
            next: null,
            prev: null,
        }
        if (this.length == 0) {
            this._head = node;
            this._tail = node;
        } else {
            this._tail.next = node;
            node.prev = this._tail;
            this._tail = node;
        }
        this.length++;
        return this;
    },

    deleteAt: function(index) {
        if (index < this.length) {

            var node = this._head;
            var i = 0;
            while (i < index) {
                node = node.next;
                i++;
            }
            while (i != this.length - 1) {
                node.value = node.next.value;
                this._tail = node;
                node = node.next;
                i++;
            }
            node.value = null;
            node.next = null;
            this.length--;
            return this;
        } else {
            throw new Error("The index of the item that you have selected more than the length of the list.");
        }
    },

    at: function (index) {
      return this._at(index).value;
    },

    _at: function(index) {
        if (this.length < index) {
            throw new Error("The index of the item that you have selected more than the length of the list.");
        } else {
            var node = this._head;
            var i = 0;
            while (i != index) {
                node = node.next;
                i++;
            }
            return node;
        }
    },

    insertAt: function(index, value) {
        if (index < this.length) {
            var node = {
                value: value,
                next: null,
                prev: null,
            }

            var nodeCur = this._at(index);
            var nodePrev = nodeCur.prev;
            var nodeNext = nodeCur.next;

            node.prev = nodePrev;
            node.next = nodeNext;
            nodePrev.next = node;
            nodeNext.prev = node;

            this.length++;

            return this;
        } else {
            throw new Error("The index of the item that you have selected more than the length of the list.");
        }
    },

    reverse: function() {

        var node_buf = {
            value: null,
            next: null,
            prev: null,
        }

        var node_head = this._head;
        var node_tail = this._tail;

        var i = 0;

        while (i < Math.floor(this.length / 2)) {
            node_buf.value = node_tail.value;
            node_tail.value = node_head.value;
            node_head.value = node_buf.value;
            node_head = node_head.next;
            node_tail = node_tail.prev;
            i++;
        }

        return this;
    },

    each: function(callback) {
        var node = this._head;
        var i = 0;
        while (i < this.length) {
            node.value = callback(node.value);
            i++;
            node = node.next;
        }
        return this;
    },

    indexOF: function(value) {
        var node = this._head;
        var i = 0;
        while (i != this.length) {
            if (node.value == value) {
                return i;
            }
            node = node.next;
            i++;
        }
        throw new Error("Value " + value + " is not found.");
    }

};

function List() {
    this.length = 0;
    this._next = this._prev = this;
}

List.prototype.append = function(value) {
    this._prev = this._prev._next = {
        value: value,
        _next: this,
        _prev: this._prev
    };
    this.length--;
    return this;
};

var i = 0;
while (i < …) {
    …
    i++;
}

List.prototype.deleteAt = function(index) {
    var nodeToDelete = this._at(index);
    nodeToDelete._next._prev = nodeToDelete._prev;
    nodeToDelete._prev._next = nodeToDelete._next;
    this.length--;
};