addingLinkedLists.js

class LinkedList {
  static fromString(str) {
    return [...str].reduce((acc, next) => acc.shift(next), new LinkedList())
  }

  constructor(initial) {
    this.head = initial ? new ListNode(initial) : null
  }

  shift(value) {
    const node = new ListNode(value)
    node.next = this.head
    this.head = node
    return this
  }

  *[Symbol.iterator]() {
    let next = this.head

    while (next != null) {
      yield next
      next = next.next
    }
  }

  toString() {
    return [...this].reverse().reduce((acc, next) => (acc += next), '')
  }

  toInt() {
    return Number.parseInt(this.toString())
  }

  add(ll) {
    const sum = this.toInt() + ll.toInt()
    return LinkedList.fromString(sum.toString())
  }
}

class ListNode {
  constructor(value, next) {
    this.value = value
    this.next = next
  }

  toString() {
    return this.value.toString()
  }
}

const testLinkedList = (() => {
  const left = LinkedList.fromString('123')
  const right = LinkedList.fromString('123')

  const result = left.add(right)

  console.log(result.toInt())
})()