在输入密码时，将密码的长度设置为6-16，且在密码长度出现小于6或是大于16时，会出现提示，效果如下图
1. 此事件为失去焦点事件，当鼠标离开密码框且点击旁边就会触发事件
2. 输入密码会出现提示，事件发生与密码的长度有关
3. 根据长度再来判断该提示的消息与效果.
代码如下：
<!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>密码框</title>
<style type="text/css">
div{
width: 600px;
margin: 100px auto;
}
.massage{
background-image: url(gth.png);
background-repeat:no-repeat;
background-position: left center;
color:black;
display: inline-block;
padding-left: 20px;
}
.right{
background-image: url(right.png);
background-repeat: no-repeat;
color: green;
}
.wrong{
background-repeat: no-repeat;
background-image: url(wrong.png);
color: red;
}
</style>
</head>
<body>
<div class="register">
<input type="password" id="pwd">
<p class="massage">请输入6-16位密码</p>
</div>
<script>
var pwd=document.querySelector('#pwd');
var aa=document.querySelector('.massage');
pwd.onblur= function(){
if(this.value.length>16 || this.value.length<6){
aa.innerText='错误，请输入6-16为密码';
aa.className='massage wrong';
}else{
aa.className='massage right';
aa.innerText='密码正确';
}
}
</script>
</body>
</html>