元素可视区判断

var div = document.querySelector('#box2');
    
    
    var timer=null
    window.onscroll= function(){
        clearTimeout(timer)
        timer = setTimeout(function(){
            var rect =div.getBoundingClientRect();
            console.log(rect)
            console.log(document.documentElement.clientWidth,document.documentElement.clientHeight)
            console.log(rect.bottom>0 && rect.bottom<document.documentElement.clientHeight+rect.height/2)
        },2000)
    }