var与let

 let btn=document.getElementById('btn')
        let result =document.getElementById('result')
        const timer={
            time:0,
            start:function (){
                //console.log(this);this指向timer对象     
                let self=this    问题描述： 为什么用var而不用let,如果是因为作用域链的原因，
                请老师帮忙分析一下使用let时的作用域链
         
                btn.addEventListener('click',function(){
                    //console.log(this);this指向btn                 
                    setInterval(function(){
                        //console.log(this); this指向window对象                       
                        self.time++
                        result.innerHTML=self.time  
                    },1000)
                },false)
            }
        }
        timer.start()