随机生成4位验证码,大写字母小写字母及数字可供随机选择(比如:nG3p)

var arr = [];
        for (var i = 48; i <= 57; i++) {
            arr.push(String.fromCharCode(i));
        }
        console.log(arr);
        for (var j = 65; j <= 90; j++) {
            arr.push(String.fromCharCode(j))
        }
        console.log(arr);
        for (var k = 97; k <= 122; k++) {
            arr.push(String.fromCharCode(k))
        }
        console.log(arr);
        var str = '';
        for (var i = 0; i < 4; i++) {
            var index = Math.floor(Math.random() * arr.length); //index代表下标。
          //利用下标找下标对应的值
            console.log(index);
            str += arr[index]
        }
        console.log(str);