实现一个红绿灯，把一个圆形 div 依照绿色 3 秒，黄色 1 秒，红色 2 秒循环扭转背景色。
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title> 红绿灯 </title>
<style>
.traffic-light {
width: 100px;
height: 100px;
border-radius: 50%;
overflow: hidden;
border: 1px solid #f1f1f1;
}
</style>
</head>
<body>
<div class="traffic-light"></div>
<script>
function keepColor(duration) {return new Promise((resolve, reject) => {setTimeout(resolve, duration);
});
}
async function changeColor (color, duration) {document.getElementsByClassName('traffic-light')[0].style.background = color;
await keepColor(duration);
}
async function run() {while (1) {await changeColor('green', 3000); // await 要嵌套，最里层有外层兴许有
await changeColor('yellow', 1000);
await changeColor('red', 2000);
}
}
run();
</script>
</body>
</html>