var checkcode = document.getElementById("code");

var btn = document.getElementById("btn");

var refresh = document.getElementById("refresh");

var code = "";

//生成随机的验证码

function setCode(){

    code = "";

    var length = 4;

    var array = [0,1,2,3,4,5,6,7,8,9,'Q','W','E','R','T','Y','U','I','O',

'P','A','S','D','F','G','H','J','K','L','Z','X','C','V','B','N','M'];

for (var i = 0;i<length;i++){

    let index = Math.floor(Math.random() * 36);

    code += array[index];

    //Math.random()代表[0.0,1.0)这个区间的取值

    // Math.floor()代表获取整数值(math.floor()向下取整，.ceil()向上取整，.round()执行标准取整（四舍五入）)

    }

    checkcode.innerHTML = code;

}

setCode();

refresh.addEventListener("click",setCode,false);