JavaScript关于DOM操作的一些小问题（2）

题目：倒计时
<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
    <style>
        #box{ 200px;height: 200px;border: 1px solid green;margin: 50px auto;line-height: 200px;text-align: center;font-size: 100px;}
        input{margin: 20px auto;display: block;}
    </style>
</head>
<body>
    <div id="box">300</div>
    <input type="button" id="start" value="开始">
    <input type="button" id="end" value="暂停">
</body>
<script>

    // 倒计时
    
    //获取页面元素
    var obox = document.getElementById("box");
    var ostart = document.getElementById("start");
    var oend = document.getElementById("end");
    //设置一个t用来每次执行计时器时清楚计时器
    var t = "";

    //开始按钮
    ostart.onclick = function(){
        //执行前清楚计时器
        clearInterval(t);
        t = setInterval(function(){
            if(obox.innerHTML == 0){
                clearInterval(t);
            }else{
                obox.innerHTML--;
            }
        },1000)
    }
    //结束按钮
    oend.onclick = function(){
        clearInterval(t);
    }

</script>
</html>