Linked List and it's methods implementations using class in JavaScript

linkedList.js

// SINGLY LINKED LIST

class SinglyLinkedList {
  constructor(head = null) {
    this.head = head;
  }

  getFirstNode() {
    return this.head;
  }

  addAtHead(val) {
    let node = {
      val,
      next: this.head,
    };

    this.head = node;
  }

  getLastNode() {
    let lastNode = this.head;

    if (this.head !== null) {
      while (lastNode.next !== null) {
        lastNode = lastNode.next;
      }
    }

    return lastNode;
  }

  addAtTail(val) {
    let node = {
      val,
      next: null,
    };

    if (this.head === null) {
      this.head = node;
      return;
    }

    let lastNode = this.getLastNode();

    lastNode.next = node;
  }

  getNodeAtIndex(index) {
    let node = this.head;

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

    return node;
  }

  addAtIndex(index, val) {
    if (index === 0) {
      this.addAtHead(val);
      return;
    }

    let node = {
      val,
      next: this.getNodeAtIndex(index),
    };

    let prevNode = this.getNodeAtIndex(index - 1);
    if (prevNode) {
      prevNode.next = node;
    }
  }

  deleteFirstNode() {
    this.head = this.head.next;
  }

  deleteLastNode() {
    if (this.head === null) {
      return;
    }

    let secondLastNode = this.head;

    while (secondLastNode.next !== this.getLastNode()) {
      secondLastNode = secondLastNode.next;
    }

    secondLastNode.next = null;
  }

  getValue(index) {
    let node = this.getNodeAtIndex(index);

    return node ? node.val : -1;
  }

  deleteAtIndex(index) {
    if (index === 0) {
      this.deleteFirstNode();
    }

    let prevNode = this.getNodeAtIndex(index - 1);
    let nextNode = this.getNodeAtIndex(index + 1);

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

let list = new SinglyLinkedList();

list.addAtHead(1); // 1
list.addAtHead(2); // 2, 1
list.addAtTail(3); // 2, 1, 3

console.log(JSON.stringify(list));