dll.js

class Node {
  constructor(val) {
    this.prev = null;
    this.next = null;
    this.value = val;
  }
}
class Dll {
  constructor() {
    this.head = null;
    this.tail = null;
    this.count = 0;
  }
  addToFirst(val) {
    var newnode = new Node(val);
    this.count++;
    if(!this.head){
      this.head = newnode;
      this.tail = newnode;
      newnode.prev = this.head;
    } else {
      this.head.prev = newnode;
      newnode.next = this.head;
      this.head = newnode;
    }
  }
  addToPos(pos, val) {
    var newnode = new Node(val);
    this.count++;
    var curr = this.head;
    var i = 1;
    while(i < pos-1) {
      curr = curr.next;
      i++
    }
    curr.next.prev = newnode;
    newnode.next = curr.next;
    newnode.prev = curr;
    curr.next = newnode;
  }
  displayFromHead() {
    var curr = this.head;
    while(curr) {
      console.log(curr.value);
      curr = curr.next;
    }
  }
  displayFromTail() {
    var curr = this.tail;
    while(curr) {
      console.log(curr.value);
      curr = curr.prev;
    }
  }
}
var dll = new Dll();
dll.addToFirst(30);
dll.addToFirst(20);
dll.addToFirst(10);
dll.addToPos(2, 50);
dll.displayFromHead();
dll.displayFromTail();