requestAnimationFrame详解

var e = document.getElementById("e");
var flag = true;
var left = 0;

function render() {
    if(flag == true){
        if(left>=100){
            flag = false
        }
        e.style.left = ` ${left++}px`
    }else{
        if(left<=0){
            flag = true
        }
        e.style.left = ` ${left--}px`
    }
}
setInterval(function(){
     render()
},1000/60)