JS上传之前获取视频时长

<style>#v{width:400px;height:300px}</style>
<video id="v" controls style="display: none;"></video><br />
<input type="file" onchange="setVideoSrc(this)" />
<script>
    function getDuration() {
        if (!isNaN(v.duration)) {
            clearInterval(timer);
            alert(v.duration)
        }
        else {
            tryTime++;
            //超过获取视频长度最大重试次数
            if (tryTime >= maxTry) { clearInterval(timer); alert('无法获取视频长度！');}
        }
    }
    var timer,tryTime,maxTry=5,delay=500;
    function setVideoSrc(f) {
        clearInterval(timer);
        var url = URL.createObjectURL(f.files[0]);
        v.src = url;
        tryTime = 0;
        timer = setInterval(getDuration, delay);//直接获取不到duration，会NaN，计时器来获取
    }
</script>