DoublyList

function Node(value) {
    this.data = value;
    this.previous = null;
    this.next = null;
}
function DoublyList() {
    this._length = 0;
    this.head = null;
    this.tail = null;
}
DoublyList.prototype.add = function(value) {
    const node = new Node(value);
    if (this._length) {
        this.tail.next = node;
        node.previous = this.tail;
        this.tail = node;
    } else {
        this.head = node;
        this.tail = node;
    }
    this._length++;
    return node;
};
DoublyList.prototype.searchNodeAt = function(position) {
    const currentNode = this.head,
        length = this._length,
        count = 1,
        message = {failure: 'Failure: non-existent node in this list.'};
    if (length === 0 || position < 1 || position > length) {
        throw new Error(message.failure);
    }
    while (count < position) {
        currentNode = currentNode.next;
        count++;
    }
    return currentNode;
};
DoublyList.prototype.remove = function(position) {
    const currentNode = this.head,
        length = this._length,
        count = 1,
        message = {failure: 'Error: non-existent node in this list.'},
        beforeNodeToDelete = null,
        nodeToDelete = null,
        deletedNode = null;
    if (length === 0 || position < 1 || position > length) {
        throw new Error(message.failure);
    }
    if (position === 1) {
        this.head = currentNode.next;
        if (!this.head) {
            this.head.previous = null;
        } else {
            this.tail = null;
        }
    } else if (position === this._length) {
        this.tail = this.tail.previous;
        this.tail.next = null;
    } else {
        while (count < position) {
            currentNode = currentNode.next;
            count++;
        }
        beforeNodeToDelete = currentNode.previous;
        nodeToDelete = currentNode;
        afterNodeToDelete = currentNode.next;
        beforeNodeToDelete.next = afterNodeToDelete;
        afterNodeToDelete.previous = beforeNodeToDelete;
        deletedNode = nodeToDelete;
        nodeToDelete = null;
    }
    this._length--;
    return message.success;
};