点击发送短信剩余秒数

<!DOCTYPE html>
<html>
<head lang="en">
    <meta charset="UTF-8">
    <title></title>
</head>
<body>
<input type="text"/>
<button id="btn">点击发送短信</button>
</body>
</html>
<script>
    var btn = document.getElementById("btn");
    var count = 5; // 数据的 10
    var timer = null; // 定时器的名字
    btn.onclick = function() {
        clearInterval(timer); // 先清除掉原来的定时器
        // alert(11);
        this.disabled = true;
         //alert(this); // this 指向的是 btn
        var that = this; // 把 btn 对象给 that var _this = this;
        timer = setInterval(sendTextMessage,1000); // 开启定时器名字 timer
        function sendTextMessage() {
            count--;
        //this.innerHTML = "还剩余"+count+"秒";
           // alert(this); // this 指向的是定时器 window
            //alert(that);
            if(count >= 0 )
            {
                that.innerHTML = "还剩余"+count+"秒";
            }
            else
            {
                that.innerHTML = "重新发送短信";
                that.disabled = false;
                clearInterval(timer); // 清除定时器
                count = 5;
            }

        }

    }
</script>