<!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>
.box{
width: 300px;
height: 30px;
margin: 0 auto;
position: relative;
border: 2px solid red;
}
.nav{
height: 100%;
background:blue;
position: absolute;
left: 0;
top: 0;
}
</style>
</head>
<body>
<div class="box">
<div class="nav">
</div>
</div>
</body>
<script>
var nav=document.querySelector(".nav");
var t;
var speed=1;
// var w= nav.offsetWidth;
t= setInterval(function(){
if(nav.offsetWidth<=300){
nav.style. width=(nav.offsetWidth)+speed+"px";
}
},30)
</script>
</html>
效果图
