Leetcode - Add Two Numbers - Two Solutions

You are given two non-empty linked lists representing two non-negative integers. The digits are stored in reverse order, and each of their nodes contains a single digit. Add the two numbers and return the sum as a linked list.

Full problem here.

Solution 1. Based on this solution

var addTwoNumbers = function(l1, l2) {
    const head = new ListNode(0)

    let firstNode = l1
    let secondNode = l2
    let currentNode = head
    let carry = 0

    while (firstNode || secondNode || carry) {
        const firstValue = firstNode?.val || 0
        const secondValue = secondNode?.val || 0