js随机生成四个数字2个字符

function generateCode(){

var chars = "ABCDEFGHIJKLMNOPQRSTUVWXTZ";?

var string_length = 6;?

var randomstring = '';?

var charCount = 0;?

var numCount = 0;?

?

for (var i=0; i<string_length; i++) {?

? ? if((Math.floor(Math.random() * 2) == 0) && numCount < 2 || charCount >= 4) {?

? ? ? ? var rnum = Math.floor(Math.random() * 10);?

? ? ? ? randomstring += rnum;?

? ? ? ? numCount += 1;?

? ? } else {?

? ? ? ? var rnum = Math.floor(Math.random() * chars.length);?

? ? ? ? randomstring += chars.substring(rnum,rnum+1);?

? ? ? ? charCount += 1;?

? ? }?

? ?}?

? }

?