<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta http-equiv="X-UA-Compatible" content="IE=edge">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>Document</title>
<style>
.bg {
background-color: yellow;
}
</style>
</head>
<body>
<input type="text" id="name"><br>
<input type="button" value="提交" id="btn">
<script>
//检测用户名是否是3-6位, 如果不满足要求高亮显示文本框
var btn = document.getElementById("btn");
var username = document.getElementById("name");
btn.onclick = function(){
//判断用户名
if(username.value.length < 3 || username.value.length > 6){
username.className = "bg";
}
else {
username.className = "";
}
};
</script>
</body>
</html>
