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。
<!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>
效果截图如下：
这个随机点名功能实现起来相对容易，利用了定时器，设置了一个开关来控制开始和暂停。
另外我在另一篇随笔中用定时器写了一个倒计时功能，实现原理和这个类似，关键的两个点是定时器和开关，（相比较而言，开关更难理解）附上地址：https://www.cnblogs.com/qiaowanze/p/11399260.html
