js 倒计时

<div id="username" onclick="start">30</div>

function start(j){
var num = $('#username').html();
//{* 倒计时开始 *}
var t = setInterval(function(){
num--;
$('#username').html(num);
//{* 倒计时到0 清除setInterval  *}
if (num < 0) {
clearInterval(t);
$('#username').html(30);
}
},1000);
}