16.JavaScript用setTimeout函数做出计时效果

JavaScript使用setTimeout函数做出计时效果：
<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>

<body>
    <button id="btn1">开始计时</button>
    <button id="btn2">结束计时</button>
    <div>0</div>
    <script>
        var num = 0;
        var timer = null;
        var div = document.querySelector("div");
        var btn1 = document.querySelector("#btn1");
        var btn2 = document.querySelector("#btn2");
        btn1.onclick = function () {
            interval(function(){
                num++;
                div.innerHTML = num;
            }, 1000);
        }
        //使用setTimeout函数实现定时器
        function interval(callback, time) {
            timer = setTimeout(function(){
                callback();
                interval(callback, time);//重新调用interval(),异步执行
            }, time);
        }
        btn2.onclick = function(){
            clearTimeout(timer);
        }
    </script>
</body>

</html>
index.html