Двусвязный список

class DoublyLinkedList {
    constructor() {
                this.size = 0;
        this.head = null;
        this.tail = null;
    }

    add(value, index) {
        const node = createNode(value);

        if (index === undefined) {
            if (this.tail) {
                node.prev = this.tail;
                this.tail.next = node;
                this.tail = node;
            } else {
                this.head = node;
                this.tail = node;
            }
        } else {
            const nodeAtIndex = this.searchByIndex(index);

            node.next = nodeAtIndex;
            node.prev = nodeAtIndex.prev;

            if (nodeAtIndex.prev) {
                nodeAtIndex.prev.next = node;
            }

            nodeAtIndex.prev = node;

            if (nodeAtIndex === this.head) {
                this.head = node;
            }
        }

        this.size++;
    }

    removeByValue(value) {
        const node = this.searchByValue(value);

        if (!node) {
            return;
        }

        this._removeNode(node);
    }

    removeByIndex(index) {
        this._removeNode(this.searchByIndex(index));
    }

    searchByIndex(index) {
        this._checkIndex(index);

        let node = this.head;

        for (let i = 0; i < index; i++) {
            node = node.next;
        }

        return node;
    }

    searchByValue(value, startIndex = 0) {
        let node = startIndex ? this.searchByIndex(startIndex) : this.head;

        while (node && node.value !== value) {
            node = node.next;
        }

        return node;
    }

    _checkIndex(index) {
        if (index >= this.size) {
            throw new Error('Индекс за пределами списка');
        }
    }

    _removeNode(node) {
        const prevNode = node.prev;
        const nextNode = node.next;

        if (prevNode) {
            prevNode.next = nextNode;
        }

        if (nextNode) {
            nextNode.prev = prevNode;
        }

        node.prev = null;
        node.next = null;

        if (node === this.head) {
            this.head = nextNode;
        }

        if (node === this.tail) {
            this.tail = prevNode;
        }

        this.size--;
    }
}

function createNode(value) {
    return {
        value,
        next: null,
        prev: null,
    };
}