如何用JavaScript实现获取验证码的效果

var code ; //在全局定义验证码        

        function createCode(){

             code = "";  

             var codeLength = 4;//验证码的长度 

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

             var random = new Array(0,1,2,3,4,5,6,7,8,9,‘A‘,‘B‘,‘C‘,‘D‘,‘E‘,‘F‘,‘G‘,

             ‘H‘,‘I‘,‘J‘,‘K‘,‘L‘,‘M‘,‘N‘,‘O‘,‘P‘,‘Q‘,‘R‘,‘S‘,‘T‘,‘U‘,‘V‘,‘W‘,‘X‘,‘Y‘,

             ‘Z‘);//随机数 

             for(var i = 0; i < codeLength; i++) {//循环操作 

                var index = Math.floor(Math.random()*36);//取得随机数的索引（0~35） 

                code += random[index];//根据索引取得随机数加到code上 

            } 

            checkCode.value = code;//把code值赋给验证码 

        }

        //校验验证码 

        function validate(){ 

            var inputCode = document.getElementById("input").value.toUpperCase();

            //取得输入的验证码并转化为大写       

            if(inputCode.length <= 0) { //若输入的验证码长度为0 

                alert("请输入验证码！"); //则弹出请输入验证码 

            }else if(inputCode != code ) { //若输入的验证码与产生的验证码不一致时 

                alert("验证码输入错误!"); //则弹出验证码输入错误 

                createCode();//刷新验证码 

                document.getElementById("input").value = "";//清空文本框 

            }else { //输入正确时 

                alert("验证通过");

            }

        }