<!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>
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