如何计算出浏览器的帧数？requestAnimationFrame

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<!DOCTYPE html>
<html lang="zh-CN">
<head>
    <meta charset="UTF-8">
    <title>帧数</title>
</head>
<body>
<div id="div"></div>
</body>
<script>

var count = 0;
var flag = true;
var cishu = 1;
function fun(){
  ++count;
  console.log("执行中");
  if(flag){
    requestAnimationFrame(arguments.callee);
  }
}
let computedZhen = fun;
computedZhen();
setInterval(function() {
  flag = false;//关闭flag是为了关闭递归
  let node = document.getElementById('div');
  ++cishu;
  node.innerText = '帧数（fps）：'+count + "
更新次数：" + cishu;
  count = 0;
  flag = true;
}, 1000);
</script>
</html>
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