JavaScript定时器的开启关闭

<html>
<head>
<meta charset="utf-8">
<title>无标题文档</title>
<style type="text/css">
    #btn1:hover,#btn2:hover {
        background: red;
    }
</style>
</head>

<body>
<input type="button" id="btn1" value="开启" />
<input type="button" id="btn2" value="关闭" />
<script>
    var obtn1=document.getElementById('btn1');
    var obtn2=document.getElementById('btn2');
    var timer=null;
    obtn1.onclick=function()
    {
        timer=setInterval(function()   //开启循环：每秒出现一次提示框
        {
            alert('a');
        },1000);
    }
    obtn2.onclick=function()
    {
        clearInterval(timer);        //关闭循环
    }
</script>
</body>
</html>

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