js实现倒计时

<!DOCTYPE HTML>
<html>
 
<head>
    <meta charset="UTF-8" />
    <title>pro.html</title>
    <style type="text/css">
    </style>
    <script>
        var x = 30,
            interval;
        window.onload = function() {
            var d = new Date("1111/1/1,0:" + x + ":0");
            interval = setInterval(function() {
                var m = d.getMinutes();
                var s = d.getSeconds();
                m = m < 10 ? "0" + m : m;
                s = s < 10 ? "0" + s : s;
                btn.innerHTML = m + ":" + s;
                if (m == 0 && s == 0) {
                    clearInterval(interval);
                    return;
                }
                d.setSeconds(s - 1);
            }, 1000);
        }
    </script>
</head>
 
<body>
    <div id="btn"></div>
</body>
 
</html>
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