probably not the best js implementation of a doubly linked list

DLinkedList.js

const DLLNode = require("./DLLNode");

module.exports = class DLinkedList {
  constructor(nodes) {
    if(!Array.isArray(nodes)) throw new TypeError(`Expected type Array<DLLNode>, got ${typeof nodes} instead`);

    this.head = null;
    this.tail = null;
    this.list = [];

    for(let i = 0; i < nodes.length; i++) {
      if(!(nodes[i] instanceof DLLNode)) throw new TypeError(`Expected type DLLNode at nodes[${i}], got ${typeof nodes[i]} instead`);

      if(i === 0) { // first node
        nodes[i].setChild(nodes[i + 1]);
        this.head = nodes[i];
        this.list.push(nodes[i]);
        continue; // break out of this loop and go to the next since this is all we can do
      }
      if(i === nodes.length - 1) { // last node
        nodes[i].setParent(nodes[i - 1]);
        this.tail = nodes[i];
        this.list.push(nodes[i]);
        continue; // break out of this loop and go to the next since this is all we can do
      }

      nodes[i].setChild(nodes[i + 1]);
      nodes[i].setParent(nodes[i - 1]);
      this.list.push(nodes[i]);
    }
  } 

  getHead() { return this.head; }

  getTail() { return this.tail; }

  getList() { return this.list; }

  push(newNode) {
    if(!(newNode instanceof DLLNode)) throw new TypeError(`Expected type DLLNode, got ${typeof newNode} instead`);

    this.getTail().setChild(newNode);
    newNode.setParent(this.getTail());
    this.tail = newNode;

    return newNode;
  }

  unshift(newNode) {
    if(!(newNode instanceof DLLNode)) throw new TypeError(`Expected type DLLNode, got ${typeof newNode} instead`);
    
    this.getHead().setParent(newNode);
    newNode.setChild(this.getHead());
    this.head = newNode;

    return newNode;
  }

  insert(index, newNode) {
    if(!(newNode instanceof DLLNode)) throw new TypeError(`Expected type DLLNode, got ${typeof newNode} instead`);
    if(typeof index !== 'number') throw new TypeError(`Expected type number, got ${typeof index} instead`); 
    
    if(index === 0) return this.unshift(newNode);
    if(index === this.getList().length - 1) return this.push(newNode);

    const parentNode = this.getList()[index - 1];
    const childNode = this.getList()[index + 1];

    parentNode.setChild(newNode);
    childNode.setParent(newNode);
    newNode.setParent(parentNode);
    newNode.setChild(childNode);

    return newNode;
  }
}

DLLNode.js

module.exports = class DLLNode {
  constructor(data) {
    this.data = data ?? null;
    this.parent = null;
    this.child = null;
  }

  store(newData) {
    this.data = newData;
    return newData;
  }

  getData() { return this.data; }

  setChild(newChild) {
    if(!(newChild instanceof DLLNode)) throw new TypeError(`Expected type DLLNode, got ${typeof newChild} instead`);

    this.child = newChild;
    return newChild;
  }

  setParent(newParent) {
    if(!(newParent instanceof DLLNode)) throw new Error(`Expected type DLLNode, got ${typeof newParent} instead`);

    this.parent = newParent;
    return newParent;
  }

  getChild() { return this.child; }

  getParent() { return this.parent; }
}

index.js

const DLinkedList = require('./DLinkedList');
const DLLNode = require('./DLLNode');

const node1 = new DLLNode('01');
const node2 = new DLLNode('02');
const node3 = new DLLNode('03');
const node4 = new DLLNode('04');

const list = new DLinkedList([node1, node2, node3, node4]);

console.log('node1 data', node1.getData());
console.log('node2 data', node2.getData());
console.log('node3 data', node3.getData());
console.log('node4 data', node4.getData());

console.log('list list', list.getList());

console.log(' ');

console.log('list head', list.getHead());
console.log('list tail', list.getTail());

console.log(' ');

console.log('list head + 1', list.getHead().getChild());
console.log('list head + 2', list.getHead().getChild().getChild());
console.log('list head + 2 - 1', list.getHead().getChild().getChild().getParent());

// untested: DLinkedList#push(), DLinkedList#unshift(), DLinkedList#insert()

For a more practical example (and maybe a better implementation?) check out a project I'm working on that I used code from here for: https://gitlab.com/akii0008/sobi