js随机点名器（简单）

附源代码;
<!DOCTYPE html>
<html lang="en">
<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>
    .box {
         200px;
        height: 200px;
        line-height: 200px;
        background-color: #ccc;
        font-size: 30px;
        text-align: center;
    }
    </style>
</head>
<body>
    <div class="box">此乃点名器</div>
    <input type="button" value="点名" id="btn">
    <!-- <button id="btn">点名</button> -->
    <script>
        var btn = document.getElementById("btn");
        var box = document.getElementsByClassName("box")[0];
// 3、写一个随机抽名的案例？（点击按钮开启定时器，
// 定时器中去随机抽取人名，把全班的人放在一个数组中，只要想办法获取随机的索引号就可以）
        btn.onclick = function() {
            if(this.value==="点名") {
            function fn() {
            var arr = ["张三","李四","王五","赵六","黑七","白八","紫九","红薯"];
            var index = parseInt(Math.random()*arr.length);
         var n1 = parseInt(Math.random()*255+1);
         var n2 = parseInt(Math.random()*255+1);
         var n3 = parseInt(Math.random()*255+1);
            box.style.background ="rgb("+n1+","+n2+","+n3+")" ;
            box.innerHTML = arr[index]; 
            } 
            this.value = "停止";
            //定时器不加var声明，涉及到作用域问题   
           timer=setInterval(fn,2);
            }else {
                clearInterval(timer);
                this.value = "点名";
            }
            
        }




        </script>
</body>
</html>
效果图如下：