Queues using Stacks

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

class Queue {
    constructor() {
        this.first = null;
        this.last = null;
        this.length = 0;
    }
    
    enqueue(value) {
        const newNode = new Node(value);
        if( this.length === 0) {
            this.first = newNode;
            this.last = newNode;
        }
        else {
            this.last.next = newNode;
            this.last = newNode;
        }
        this.length++;
    }
    
    peek() {
        return this.first;
    }
    
    dequeue() {
        if(this.length === 0) {
            return null;
        }
        if(this.first === this.last) {
            this.last = null;
        }
        const temp = this.first;
        this.first = this.first.next;
        this.length--;
        return temp;
    }
    
    isEmpty() {
        return (this.length === 0) ? true : false;
    }
}