class.js

let that;
class Tab{
   constructor(el){
       that = this;
       this.main = document.querySelector(el);
       this.button = document.querySelector("button");
       this.ul = document.querySelector("ul");
       this.lis = document.querySelectorAll("li");
       this.init();
   }
   getRoot(){
       console.log(this.main);
   }
   init(){
       this.lis.forEach( li => {
           li.onclick = function (li){
               this.style.display = "none";
           } 
       });
       this.button.onclick = this.addTab;
   }
   addTab(){
       var li = document.createElement("li");
       li.innerText = "ok";
       that.ul.appendChild(li);
   }
}
var tab = new Tab("#tab");
tab.getRoot();