问题如题，代码如下：
请问为什么当我点击了鼠标之后，虽然出发了js中的click函数，但是转瞬即逝，又返回之前的页面了？
<!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>
.bgc {
display: none;
position: absolute;
top: 0;
width: 100%;
height: 100%;
background-color: gray;
z-index: -1;
}
label {
display: inline-block;
width: 100px;
height: 35px;
text-align: center;
}
.header a {
display: block;
text-align: center;
text-decoration: none;
font-size: 30px;
color: black;
}
.login {
display: none;
width: 500px;
height: 300px;
/* border: 2px solid black; */
position: absolute;
top: 50%;
left: 50%;
transform: translate(-50%,-50%);
background-color: pink;
}
.login-header {
text-align: center;
margin-top: 20px;
font-size: 20px;
position: relative;
}
.login-header a {
text-decoration: none;
border: 3px solid black;
font-size: 15px;
color: red;
position: absolute;
right: 0px;
top: -20px;
}
.login-body {
margin: 70px;
/* border: 1px solid red; */
}
.login-input {
margin: 10px;
font-size: 20px;
}
.login-button {
text-align: center;
}
input {
height: 30px;
width: 190px;
}
.login-button button a {
text-decoration: none;
font-size: 20px;
color: red;
}
</style>
</head>
<body>
<div class="header"><a href="">点击，弹出登录框</a></div>
<div class="login">
<div class="login-header">登陆会员
<span><a href="" class="closeBtn">关闭</a></span>
</div>
<div class='login-body'>
<div class='login-input'>
<label for="">用户名:</label>
<input type="text" placeholder="请输入用户名">
</div>
<div class='login-input'>
<label for="">密码:</label>
<input type="password" placeholder="请输入密码">
</div>
<div class="login-button"><button><a href="">登录</a></button></div>
</div>
</div>
<div class="bgc"></div>
<script>
var header = document.querySelector('.header');
var login = document.querySelector('.login');
var bgc = document.querySelector('.bgc');
header.addEventListener('click',function() {
login.style.display = 'block';
bgc.style.display = 'block';
})
</script>
</body>
</html>
