Aden Eilers

Linked List in JavaScript

Written by Aden Eilers on

javascript algorithms

What is a Linked List?
A linked list data structure involves a series of Nodes linked together. Each Node will have a data value and a reference to the next Node in the list. In the last Node, the reference will be set to null. Linked lists are not commonly used in front end web development, but they are still very popular for interview coding problems.

Implementation of a Linked List with a few helper methods

class Node {
  constructor(data, next = null) {
    this.data = data;
    this.next = next;
  }
}

class LinkedList {
  constructor() {
    this.head = null;
  }

  // Inserts a Node into the Linked List
  insertFirst(data) {
    this.head = new Node(data, this.head);
  }

  // Returns the number of nodes
  size() {
    let node = this.head;
    let count = 0;

    while (node) {
      count++;
      node = node.next;
    }

    return count;
  }

  // Returns the first Node
  getFirst() {
    return this.head;
  }

  // Returns the last Node
  getLast() {
    if (!this.head.next) {
      return null;
    }
    let node = this.head;
    while (node) {
      if (!node.next) {
        return node;
      }
      node = node.next;
    }
  }

  // Removes all Nodes from the Linked List
  clear() {
    this.head = null;
  }

  // Removes the first Node from the Linked List
  removeFirst() {
    if (this.head) {
      this.head = this.head.next;
    }
  }
}

Here it is in use:

let list = new LinkedList();
list.insertFirst(1);
list.insertFirst(2);
list.insertFirst(3);
// list = {
//   head: {
//     data: 3,
//     next: {
//       data: 2,
//       next: {
//         data: 1,
//         next: null
//       }
//     }
//   }
// }

list.getFirst(); // { data: 3, next:... }
list.getLast(); // { data: 1, next: null }
list.size(); // 3
list.removeFirst(); // { head: { data: 2, next:... }}
list.clear(); // { head: null }

You will notice that a Linked List in JavaScript is simply a series of nested objects. The list will always start with a head, and the last node reference will be null.

If you are preparing for coding interviews, here are a few coding challenges involving Linked Lists