Queue.js

import LinkedList from '../linked-list/LinkedList';

export default class Queue {
 constructor() {
 // We're going to implement Queue based on LinkedList since the two
 // structures are quite similar. Namely, they both operate mostly on
 // the elements at the beginning and the end. Compare enqueue/dequeue
 // operations of Queue with append/deleteHead operations of LinkedList.
 this.linkedList = new LinkedList();
 }

 /**
 * @return {boolean}
 */
 isEmpty() {
 return !this.linkedList.head;
 }

 /**
 * Read the element at the front of the queue without removing it.
 * @return {*}
 */
 peek() {
 if (this.isEmpty()) {
 return null;
 }

 return this.linkedList.head.value;
 }

 /**
 * Add a new element to the end of the queue (the tail of the linked list).
 * This element will be processed after all elements ahead of it.
 * @param {*} value
 */
 enqueue(value) {
 this.linkedList.append(value);
 }

 /**
 * Remove the element at the front of the queue (the head of the linked list).
 * If the queue is empty, return null.
 * @return {*}
 */
 dequeue() {
 const removedHead = this.linkedList.deleteHead();
 return removedHead ? removedHead.value : null;
 }

 /**
 * @param [callback]
 * @return {string}
 */
 toString(callback) {
 // Return string representation of the queue's linked list.
 return this.linkedList.toString(callback);
 }
}