ll.js

class Node {
  constructor(data, next_node) {
    this.data = data           || null
    this.next_node = next_node || null
  }
}

class List {
  constructor() {
    this.head = new Node
  }
  
  tail(node) {
    node = node || this.head
    if (node.next_node === null) return node
    return this.tail(node.next_node)
  }
  
  append(new_data) {
    return this.tail().next_node = new Node(new_data)
  }
  
  preppend(new_data) {
    let new_list = new List
    new_list.append(new_data)
    new_list.tail().next_node = this.head.next_node
    this.head = new_list.head
  }
}

//
const l = new List
//

//
l.append("wow")
l.append("lol")
//

// TEST APPEND AND TAIL WORK AS EXPECTED
if (l.tail().data !== "lol") {
  console.log("TAIL TEST FAILED")
} else {
  console.log("TAIL TEST PASSED")
}

//
l.preppend("omg")
// 

// TEST PREPPEND DOES NOT CHANGE THE TAIL
if (l.tail().data !== "lol") {
  console.log("TAIL TEST FAILED")
} else {
  console.log("TAIL TEST PASSED")
}

// TEST PREPPEND ACTUALLY PREPENDS
if (l.head.next_node.data !== "omg") {
  console.log("TAIL TEST FAILED")
} else {
  console.log("TAIL TEST PASSED")
}