用javascript写的字符串加密程序运行出错

Question

编程思路：

1、由用户输入要加密的字符串（仅允许小写字母不包含空格）

2、把字符串中的每一个字母按字母表顺序向后移动两位输出新字符串

注释，暂不考虑“溢出”问题（即用户输入了x以后的字母），代码如下：

<script>   
//用split()函数把用户输入的字符串分割并赋值给数组str[]
var str=prompt("请输入要加密的字符串:").split(""); 

//定义包含26个小写字母的alphabet数组
var alphabet=["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"];

//定义alphabet数组的索引变量i
for(var i=0; i<=25;i++)

//定义str数组的索引变量n
for(var n=0; n<=str.length; n++)

//判断str数组中元素在alphabet中的位置，并往后移2位
if(str[n]==alphabet[i])
str[n]=alphabet[i+2]

//用jon()函数把str数组转换成字符串并赋值给newStr
var newStr=[str[n]].join("");

//输出结果
alert("加密后的新字符串是："+newStr);
</script>

已经很晚了，我没时间翻书再检查错误了，明天一早我还要去工地搬砖，恳请列位大侠直接帮忙修改成能正确运行的代码，我明天抽空拜研之

Answer 1

var str=prompt("请输入要加密的字符串:").split("");
var newStr=[];
var alphabet=["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"];
for(var n=0; n<=str.length; n++)
for(var i=0; i<=25;i++)
if(str[n]==alphabet[i])
newStr[n]=alphabet[i+2];

alert("加密后的新字符串是："+newStr.join(""))