js实现读秒

var num = 3;
var timer = null;
$('#window').html(num+ '秒后干啥');
clearInterval(timer);
timer = setInterval(function(){
    num--;
    if(num<1) {
        clearInterval(timer);
        window.location.href = ""
    }
    $('#window').html(num+'秒后干啥');
},1000);