原生JS实现简易随机点名功能

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定时器的工作原理，这里将用引用How JavaScript Timers Work中的例子来解释定时器的工作原理，该图为一个简单版的原理图。·

上图中，左侧数字代表时间，单位毫秒；左侧文字代表某一个操作完成后，浏览器去询问当前队列中存在哪些正在等待执行的操作；蓝色方块表示正在执行的代码块；右侧文字代表在代码运行过程中，出现哪些异步事件。该图大致流程如下：

程序开始时，有一个JS代码块开始执行，执行时长约为18ms，在执行过程中有3个异步事件触发，其中包括一个setTimeout、鼠标点击事件、setInterval
第一个setTimeout先运行，延迟时间为10ms，稍后鼠标事件出现，浏览器在事件队列中插入点击的回调函数，稍后setInterval运行，10ms到达之后，setTimeout向事件队列中插入setTimeout的回调
当第一个代码块执行完成后，浏览器查看队列中有哪些事件在等待，他取出排在队列最前面的代码来执行
在浏览器处理鼠标点击回调时，setInterval再次检查到到达延迟时间，他将再次向事件队列中插入一个interval的回调，以后每隔指定的延迟时间之后都会向队列中插入一个回调
后面浏览器将在执行完当前队头的代码之后，将再次取出目前队头的事件来执行

这里只是对定时器的原理做一个简单版的描述，实际的处理过程比这个复杂。

下面我们利用定时器实现一个简单的随机点名功能，代码如下：

<!DOCTYPE html>

<html lang="zh">

<head>

    <meta charset="UTF-8" />

    <meta name="viewport" content="width=device-width, initial-scale=1.0" />

    <meta http-equiv="X-UA-Compatible" content="ie=edge" />

    <title>Document</title>

    <style>

        div{width: 300px;height: 100px;margin: 20px auto;font-size: 50px;text-align: center;line-height: 100px;}

        #btn{width: 200px;height: 40px;margin: 10px auto;background: red;color: #FFFFFF;font-size: 20px;text-align: center;cursor: pointer;display: block;}

    </style>

</head>

<body>

    <div id="content">请点击按钮</div>

    <input type="button" id="btn" value="开始">

    <script>

        var ocontent = document.getElementById("content");

        var obtn = document.getElementById("btn");

        var onoff = 1;        //打开

        var timer = null;        //设置全局变量

        var str = ["亚瑟","鬼谷子","铠","橘右京","项羽","梦奇","白起","赵云","李白","韩信","刘备","鲁班七号","墨子","孙膑","周瑜","庄周","廉颇","程咬金","典韦","后羿","扁鹊","李元芳","张飞","刘禅","兰陵王","达摩","曹操","钟馗","高渐离","宫本武藏","吕布","嬴政","姜子牙","老夫子","狄仁杰","夏侯惇","关羽","哪吒","太乙真人","干将莫邪","成吉思汗","牛魔","百里守约","百里玄策","苏烈","黄忠","诸葛亮","东皇太一","杨戬","后羿","孙悟空","张良","韩信","刘邦","达摩","马可波罗","娜可露露","露娜","妲己","甄姬","虞姬","大乔","小乔","王昭君","貂蝉","芈月","阿珂","花木兰","武则天","不知火舞","孙尚香","蔡文姬","安琪拉","钟无艳","女蜗","雅典娜","艾琳"]

        obtn.onclick = function(){

            if(onoff == 1){        //开始随机点名

                clearInterval(timer);//避免重复点击按钮出现bug

                timer = setInterval(function(){

                    var randomNum = Math.round(Math.random()*(str.length-1))

                    ocontent.innerHTML = str[randomNum];

                },10)

                onoff = 0;    //"定位"到暂停功能

                obtn.value = "就是你了"

            }else if(onoff == 0){

                clearInterval(timer);

                onoff = 1;    //暂停后需恢复到重新开始的功能

                obtn.value = "继续"

            }

        }

    </script>

</body>

</html>