js特效遮罩层（弹出层）

<!DOCTYPE html>
<html>

<head>
<meta charset="UTF-8">
<title></title>
<style>
* {
margin: 0px;
padding: 0px;
}

.mask {
100%;
/*height: 500px;*/
position: fixed;
top: 0px;
left: 0px;
background-color: black;
opacity: 0.5;
}

.show {
500px;
height: 300px;
position: fixed;
top: 100px;
left: 300px;
background-color: white;
z-index: 999;
}
</style>
</head>

</html>
<script>
var mask = document.getElementsByClassName('mask')[0];
var show = document.getElementsByClassName("show")[0];

var c_height = document.documentElement.clientHeight;
var c_width = document.documentElement.clientWidth;

mask.style.height = c_height + "px";
show.style.left = c_width / 2 - 250 + "px";
show.style.top = c_height / 2 - 150 + "px";

function show_plus() {
mask.removeAttribute("hidden");
show.removeAttribute("hidden");
}

mask.onclick = function() {
mask.setAttribute("hidden", "hidden");
show.setAttribute("hidden", "hidden");
}

window.onresize = function() {
var c_height = document.documentElement.clientHeight;
var c_width = document.documentElement.clientWidth;

mask.style.height = c_height + "px";
show.style.left = c_width / 2 - 250 + "px";
show.style.top = c_height / 2 - 150 + "px";
}
</script>