stack.js

class Node{
    constructor(val){
        this.val = val;
        this.next = null;
    }
}
class Stack{
    constructor(){
        this.first = null;
        this.last = null;
        this.size = 0;
    }
    push(val){
        var newNode = new Node(val);
        if(!this.first){
            this.first = newNode;
            this.last = newNode;
        }else{
            var temp = this.first;
            this.first = newNode;
            this.first.next = temp;
        }
        return ++this.size;
    }
    pop(){
        if(!this.first) return null;
        var temp = this.first;
        if(this.size===1){
            this.last = null;
        }
        this.first = this.first.next;
        this.size--;
        return temp;
    }
}
var stack = new Stack();
stack.push(1);
stack.push(2);
stack.push(3);
stack.push(4);
stack.pop();
console.log(stack);