<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<meta http-equiv="X-UA-Compatible" content="ie=edge">
<title>Document</title>
<style>
* {
padding: 0;
margin: 0;
list-style: none;
}
#aaa { width: 100px; height: 100px; background: aqua;
position: absolute;
top: 0px;
left:0px;}
</style>
<script>
window.onload= function(){
var aaa = document.getElementById("aaa")
var num = 0
var sudu =0
setInterval( fn, 20)
function fn(){
num += sudu
aaa.style.left = num + 'px'
if(num <= 0){ sudu = +5 }
if (num >=500){ sudu = -5 }
}
}
</script>
</script>
</script>
</head>
<body>
<div id = "aaa"> </div>
</body>
</html>
