Stack and Queue data structure in JavaScript

Stack and Queue.js

//Stack implementation==================================================================================
// Stack is a container of objects that are inserted and removed according to the last-in first-out (LIFO) principle. 
class Node {
  constructor(data, next = null) {
    this.data = data;
    this.next = next;
  }
}

class Stack {
  constructor() {
    this.first = null;
    this.last = null;
    this.size = 0;
  }
  //Adds element to the beginning of the list. similar to Array.unShift()
  add(data) {
    const newNode = new Node(data);
    if (!this.first) {
      this.last = newNode;
      this.first = newNode;
    } else {
      const cptrFirst = this.first;
      this.first = newNode;
      newNode.next = cptrFirst;
    }
    return ++this.size;
  }

  //Removes element from the beginning of the list. similar to Array.shift()
  remove() {
    if (!this.first) return null;
    const cptrFirst = this.first;
    this.first = this.first.next;
    this.size--;
    cptrFirst.next = null; //removing the connection, not necessary but important
    return cptrFirst.data;
  }
}
//This class two methods are similar to `Array.unShift()` and `Array.shift()`, but it can be also done with `Array.push()` and `Array.pop()`

//EXAMPLES
const stack = new Stack();
stack.add("Hello");
stack.add("World");
stack.add("!");
stack.remove()







//Queue is a container of objects (a linear collection) that are inserted and removed according to the first-in first-out (FIFO) principle.

//Queues implementation==========================================================================
class Node {
  constructor(data, next=null) {
    this.value = data;
    this.next = next;
  }
}

class Queues {
  constructor() {
    this.first = 0;
    this.last = 0;
    this.size = 0;
  }

  //Adds element to the end of the list. Similar to Array.push()
  add(data) {
    const newNode = new Node(data);
    if (!this.first) {
      this.first = newNode;
    } else {
      //`this.tail.next` has the same reference as `this.head`'s very last item aka the last item of the list. So modifying `this.tail.next` will also modifies `this.head`'s last item.
      this.last.next = newNode;
    }
    this.last = newNode;
    return ++this.size;
  }

  //removes element from the beginning of the list. Similar to Array.shift()
  remove() {
    if (this.size === 0) return null;
    if (this.size === 1) this.last = null;
    const cptrFirst = this.first;
    this.first = this.first.next;
    this.size--;
    cptrFirst.next = null; //removing the connection, not necessary but important.
    return cptrFirst;
  }
}

//This class have two method which are similar to `Array.push()` and `Array.shift()`, but it can be also done with creating methods like `Array.unShift()` and `Array.pop()`

//EXAMPLES
const queues = new Queues();
queues.add("Hello");
queues.add("World");
queues.add("!");