text: 
  class Stack{
    constructor(){
      this.arr = [];
    }
    push(val){
      this.arr.push(val);
      return this.arr;
    }
    isEmpty(){
      return this.arr.length === 0;
    }
    pop(){
      if(this.arr === 0){
        return null;
      }
      return this.arr.pop();
    }
    peek(){
      if(this.arr.length === 0){
        return null;
      }
      return this.arr[this.arr.length-1];
    }
  }
  
  module.exports = Stack;