Untitled

<!DOCTYPE html>
<html lang="en">
<head>

<script>
function div1_opacity() {
    if(document.getElementById("div1").style.opacity==0)
        document.getElementById("div1").style.opacity=1
    else
        document.getElementById("div1").style.opacity=0;
}
</script>

</head>

<body>

    <h1>test</h1>

    <div id=div1 style="height: 100px; width: 100px; background-color: blue; opacity: 1;" >   div1  </div>

    <button id=butt1 onclick=div1_opacity()> go </button>

</body>

</html>