js中随机生成4位数的验证码，要有字母和数字

代码：

function code(){
            let num1 = Math.floor(Math.random()*10)
            let num2 = Math.floor(Math.random()*10)
            let letter1 = String.fromCharCode(Math.floor(Math.random() * 26) + 97);
            let letter2 = String.fromCharCode(Math.floor(Math.random() * 26) + 65);
            var arr = [num1,num2,letter1,letter2]
            var str = "";
            for(var i=0;i<4;i++){
                str += arr[Math.floor(Math.random()*(arr.length))]
            }
            return str
        }
        document.write(code());

效果图：