开关灯

<!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>开关灯</title>
</head>

<body>
    <button id="btn">开关灯</button>
    <script>var btn = document.getElementById('btn');
        var flag = 0;
        btn.onclick = function () {
            if (flag == 0) {
                document.body.style.backgroundColor = 'black';
                flag = 1;
            } else {
                document.body.style.backgroundColor = '#fff'
                flag = 0;
            }
        }</script>
</body>

</html>