JavaScript隨機四位驗證碼

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

<button id="btn1" onclick = "fn1()">隨機生成驗證碼</button>
<div id = "fn1"></div>

　　

//幾個常見字母的ASCII碼大小： “A”為65；“a”為97；“0”為 48；

var oBtn1 = document.getElementById('btn1');
var oFn1 = document.getElementById('fn1');
var fn1 = function(){
var arr = [];  //創建空數組，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++) { //從0-61之間取隨機數
var index = Math.floor(Math.random() * 62); //返回 0 至 61 之間的數
var char = String.fromCharCode(arr[index]);//轉換ASCII碼
arr2.push(char);
}
var str1 = arr2.toString(); //數組轉換字符串
var str2 = X.replace(/,/g, "");
oFn1.innerHTML = str2;
}