Linked list implementation of stack adding and removing to the beginning for constant time

app.js

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

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

    push(value){
        const newNode = new Node(val);
        //if stack is empty
        if (!this.first) {
          this.first = newNode;
          this.last = newNode;
          // add current first pointer to new first(new node), and make new node new first
        } else {
          newNode.next = this.first;
          this.first = newNode;
        }
        //add 1 to size
        this.size++;

        return this;
    }
    pop(){
        //if stack is empty return false
        if (this.size === 0) return false;
        //get poppednode
        const poppedNode = this.first;
        //get new first (could be NULL if stack is length 1)
        const newFirst = this.first.next;
        //if newFirst is null, reassign last to newFirst(null)
        if (!newFirst) {
            this.last = newFirst;
        }
        //assign new first
        this.first = newFirst;
        //remove reference to list
        poppedNode.next = null;
        //remove 1 from size
        this.size--;
        //return poppednode
        return poppedNode;
        }
    log(){
           let currentNode = this.first
           let i = 0;
           while(currentNode){
               console.log(i, currentNode.value)
               i++;
               currentNode = currentNode.next
           }
        }
}

const studentStack = new Stack; 
studentStack.push({"Willow Rosenberg": "A"});
studentStack.log();
//[{ "Willow Rosenberg": "A" }]

studentStack.push({"Xander Harris": "C"});
//[{ "Willow Rosenberg": "A" }, { "Xander Harris": "C" }]
studentStack.log();

studentStack.push({"Cordelia Chase": "B+"});
studentStack.log();
//[{ "Willow Rosenberg": "A" },{ "Xander Harris": "C" },{ "Cordelia Chase": "B+" }]

studentStack.push({"Buffy Summers": "B"});
studentStack.log();
//[{ "Willow Rosenberg": "A" },{ "Xander Harris": "C" },{ "Cordelia Chase": "B+" },{ "Buffy Summers": "B" }]


const firstOut = studentStack.pop();
// { "Buffy Summers": "B" }
studentStack.log();
//[{ "Willow Rosenberg": "A" },{ "Xander Harris": "C" },{ "Cordelia Chase": "B+" }]

const secondOut = studentStack.pop();
//{"Cordelia Chase": "B+"}
studentStack.log();
//[{ "Willow Rosenberg": "A" }, { "Xander Harris": "C" }]