js字符串等距替换加密

代码：

var str = "abcdxyzABCXYZ";    //    str为需要加密的字符串，encryptLen为等距的距离长度，体重A->D则encryptLen=3    function encryption(str, encryptLen) {        var arr = str.split('');        var arrLen = arr.length;        for (var i = 0; i < arrLen; i++) {            var charNum = arr[i].charCodeAt(0);            if (charNum > 96) {                if (charNum > 119) {                    arr[i] = String.fromCharCode(charNum - 23);                } else {                    arr[i] = String.fromCharCode(charNum + encryptLen);                }            } else {                if (charNum > 87 && charNum < 91) {                    arr[i] = String.fromCharCode(charNum - 23);                } else {                    arr[i] = String.fromCharCode(charNum + encryptLen);                }            }        }        return arr.join('');    }    console.log(encryption(str, 3));