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 = newNode;
           this.last = newNode;
        } else {
            this.last.next = newNode;
            this.last = newNode;
        }
        
        this.length++;
        return this;
    }
    
    dequeue(){
        if(this.isEmpty()) return null;
        if(this.first === this.last) this.last = null;
        
        const removedNode = this.first;
        this.first = this.first.next;
        this.length--;
        
        return removedNode;
    }
    
    isEmpty(){
        return this.length === 0;
    }
}