JavaScript随机四位验证码

<style>
div{width:100px;height:50px;border:1px solid #000;text-align:center;line-height:50px;}
</style>

<button id="btn1" onclick = "fn1()">随机生成验证码</button>
<div id = "fn1"></div>

//几个常见字母的ASCII码大小： “A”为65；“a”为97；“0”为 48；

var oBtn1 = document.getElementById(‘btn1‘);
var oFn1 = document.getElementById(‘fn1‘);
var fn1 = function(){
var arr = [];  //创建空数组，放入a~Z 0~9
for(var i=65;i<=90;arr.push(i),i++){} //a~z
for(var j=97;j<=122;arr.push(j),j++){}//A~Z
for(var k=48;k<=57;arr.push(k),k++){}//0~9
var arr2 = [];
for(var i=0;i<4;i++) { //从0-61之间取随机数
var index = Math.floor(Math.random() * 62); //返回 0 至 61 之间的数
var char = String.fromCharCode(arr[index]);//转换ASCII码
arr2.push(char);
}
var str1 = arr2.toString(); //数组转换字符串
var str2 = X.replace(/,/g, "");
oFn1.innerHTML = str2;
}