为什么start()函数里写成 “var...

为什么start()函数里写成 “var i=setInterval(clock,1000)；” 就不行了？

<!DOCTYPE html><html><head> <meta charset="UTF-8"> <title>Document</title><script type="text/javascript">var i=setInterval(clock,1000);function clock() {var mytime =new Date();var year =mytime.getFullYear();var month =mytime.getMonth();var date =mytime.getDate();var week =mytime.getDay();var weeks =["日","一","二","三","四","五","六"];var day =weeks[week];var hour =mytime.getHours();var minute =mytime.getMinutes();var second =mytime.getSeconds();time_now =year+"年"+(month+1)+"月"+date+"日"+"星期"+day+hour+":"+minute+":"+second;document.getElementById('txt').value=time_now;}function start() {i=setInterval(clock,1000); /*我的问题：为什么如果这行写成“var i=setInterval(clock,1000)"，第二次单击stop时stop按钮就失效了呢？为嘛非得去掉这个"var"?*/}</script></head><body><input type="text" id="txt" style="width:200px"><input type="button" onclick="start()" value="start"><input type="button" onclick="clearInterval(i)" value="stop"></body></html>

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