Random four JavaScript codes

<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>

　　

// ASCII code size of several common letters: "A" is 65; "a" 97; "0" is 48; 

var oBtn1 = document.getElementById ( 'btn1'); 
var oFn1 = document.getElementById ( 'Fn1 '); 
var Fn1 = function () { 
var ARR = []; // create empty array into the Z 0 ~ ~. 9 A 
for (var = I 65; I <= 90; arr.push (I), I ++ )} {A ~ Z // 
for (var J = 97; J <= 122; arr.push (J), J ++) {} // the Z A ~ 
for (var = K 48; K <= 57 is; ARR. Push (K), K ++) {} // ~ 0. 9 
var arr2 is = []; 
for (var I = 0; I <. 4; I ++) {// taken from a random number between 0-61 
var index = the Math. floor (Math.random () * 62) ; // returns the number between 0 and 61 is 
var char = the String.fromCharCode (ARR [index]); // convert ASCII code 
arr2.push (char); 
} 
var str1 = arr2.toString (); // convert the string array 
var str2 X.replace = (/, / G, ""); 
oFn1.innerHTML = str2; 
}