while循环获取验证码

var str='qwertyuioplkjhgfdsazxcvbnmMNBVCXZASDFGHJKLPOIUYTREWQ0123456789'
function getcode(){
    var index='';
    while(index.length<4){
        var num=str[Math.round(Math.random()*(str.length-1))];
        if(index.indexOf(num)==-1){
            index+=num;
        }
    }
    return index;
}
getcode()
console.log(getcode())