Разворот односвязного списка

ОБновите ваш head как то так

public void reverse(){
    Node currNode = head;
    Node prevNode = null;

    while (currNode != null){
        Node next = currNode.next;
        currNode.next = prevNode;

        prevNode = currNode;
        currNode = next;
    }

    head = prevNode;
}

interface ILinkedListNode<T> {
    value: T;
    next: ILinkedListNode<T> | null;
    hasNext: boolean
}

class LinkedListNode<T> implements ILinkedListNode<T> {
    value: T;
    next: ILinkedListNode<T> | null;
    hasNext: boolean;
    constructor(props: T, prev: ILinkedListNode<T> | null) {
        this.value = props;
        this.next = null
        this.hasNext = false
        if (prev) {
            prev.next = this;
            prev.hasNext = true
        }
    }
}

class LinkedList<T> implements ILinkedList<T> {
    private _size: number = 0
    private firstItem: ILinkedListNode<T> | null = null
    private lastItem: ILinkedListNode<T> | null = null
    constructor(props?: T | null) {
        props && this.add(props)
    }

 add(value: T): void {
        this._size++;
        if (!this.firstItem) {
            this.firstItem = new LinkedListNode(value, null)
            this.lastItem = this.firstItem
        } else {
            const item = new LinkedListNode(value, this.lastItem);
            this.lastItem = item;
        }
    }

public reverse(): ILinkedList<T> {
    let [prev, middle, next] = [
        this.firstItem, this.firstItem?.next, this.firstItem?.next?.next];
        if (!prev) {
            return new LinkedList<T>();
        }

        if (!middle) {
            return new LinkedList<T>(prev.value);
        }

        if (!next) {
            const linkedList = new LinkedList(middle.value);
            linkedList.add(prev.value)
            return linkedList
        }

        this.lastItem = prev;
        this.lastItem.next = null;
        this.lastItem.hasNext = false

        for (; next;) {
            middle.next = prev;
            middle.hasNext = true;

            prev = middle;
            middle = next;

            next = next.next

            if (!next) {
                this.firstItem = middle;
                this.firstItem.next = prev;
                this.firstItem.hasNext = true;
            }
        }
        return this
    }
}

// проверка
const arr = [1, 2, 3, 4, 5, 6, 7]
const linkedList = new LinkedList()
arr.forEach(el => {
    linkedList.add(el)
})
linkedList.reverse();
linkedList.forEach(el => console.log('forEach in reverse', el))