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JS计时器

<!doctype html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport"
          content="width=device-width, user-scalable=no, initial-scale=1.0, maximum-scale=1.0, minimum-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
</head>
<body>

<div>当前时间为:<span id="timeNow">00:00:00</span></div>
<button id="timeBegin">计时开始</button>
<button id="timeEnd">计时结束</button>
<button id="timeClear">计时清除</button>

<script>
    //定义初始值 计时器
    var timer = "";
    //开始计时
    var beginBtn = document.getElementById("timeBegin");
    var endBtn = document.getElementById("timeEnd");
    var clearBtn = document.getElementById("timeClear");

    var min,second,millisecond;//分  秒  毫秒
    min= second=millisecond=0; //初始化


    function time() {
        millisecond+=50;
        if (millisecond >= 1000) {
            millisecond=0
            second += 1;
        }
        if (second >= 60) {
            second=0
            min += 1;
        }
        counts = min + ':' + second + ':' + millisecond;
        document.getElementById("timeNow").innerText = counts;
    }

    //开始计时
    function BeginTime() {
        beginBtn.onclick = function () {
            timer = setInterval(time, 50);

        }
    }
    //结束计时
    function EndTime() {
        endBtn.onclick = function () {
            clearInterval(timer);
        }
    }
    //计时清除
    function ClearTime() {
        clearBtn.onclick = function () {
            document.getElementById("timeNow").innerHTML = "00:00:00";
        }
    }

    BeginTime();
    EndTime();
    ClearTime()


</script>
</body>
</html>