実装
コピペで動くはず
<!DOCTYPE html>
<html>
<head>
<meta name="viewport" content="width=device-width, initial-scale=1">
<style>
#overlay {
position: fixed;
display: none;
width: 100%;
height: 100%;
top: 0;
left: 0;
right: 0;
bottom: 0;
background-color: rgba(0,0,0,0.5);
z-index: 2;
cursor: pointer;
}
</style>
</head>
<body>
<div id="overlay" onclick="off()"><h1>Hay</h1></div>
<button onclick="on()">Turn on overlay effect</button>
<script>
function on() {
document.getElementById("overlay").style.display = "block";
}
function off() {
document.getElementById("overlay").style.display = "none";
}
</script>
</body>
</html>