Queue.js

// Implementing a queue
class Node {
  constructor(value) {
    this.value = value;
    this.next = null;
  }
}

class Queue {
  constructor() {
    this.first = null;
    this.last = null;
    this.length = 0;
  }

  peek() {
    return this.first;
  }

  enqueue(value) {
    const newNode = new Node(value);
    if (this.isEmpty()) {
      this.first = this.last = newNode;
      return this;
    }
    const follower = this.last;
    this.last = newNode;
    this.last.next = follower;
    this.length++;
    return this;
  }

  dequeue() {
    if (!this.first) {
      return null;
    }
    if (this.first === this.last) {
      this.last = null;
    }
    this.first = this.first.next;
    this.length--;
    return this;
  }

  isEmpty() {
    if (this.first === null) return true;
    return false;
  }
}

const myQueue = new Queue();
myQueue.enqueue(1);
myQueue.enqueue(2);
myQueue.enqueue(3);
myQueue.enqueue(4);
myQueue.enqueue(5);

// Implementing a queue using a stack (array methods)
class CrazyQueue {
  constructor() {
    this.first = [];
    this.last = [];
  }

  enqueue(value) {
    const length = this.first.length;
    for (let i = 0; i < length; i++) {
      this.last.push(this.first.pop());
    }
    this.last.push(value);
    return this;
  }

  dequeue() {
    const length = this.last.length;
    for (let i = 0; i < length; i++) {
      this.first.push(this.last.pop());
    }
    this.first.pop();
    return this;
  }
  peek() {
    if (this.last.length > 0) {
      return this.last[0];
    }
    return this.first[this.first.length - 1];
  }
}