var keyStr = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/=";
//将Ansi编码的字符串进行Base64编码
function encode64(input) {
var output = "";
var chr1, chr2, chr3 = "";
var enc1, enc2, enc3, enc4 = "";
var i = 0;
do {
chr1 = input.charCodeAt(i++);
chr2 = input.charCodeAt(i++);
chr3 = input.charCodeAt(i++);
enc1 = chr1 >> 2;
enc2 = ((chr1 & 3) << 4) | (chr2 >> 4);
enc3 = ((chr2 & 15) << 2) | (chr3 >> 6);
enc4 = chr3 & 63;
if (isNaN(chr2)) {
enc3 = enc4 = 64;
} else if (isNaN(chr3)) {
enc4 = 64;
}
output = output + keyStr.charAt(enc1) + keyStr.charAt(enc2)
+ keyStr.charAt(enc3) + keyStr.charAt(enc4);
chr1 = chr2 = chr3 = "";
enc1 = enc2 = enc3 = enc4 = "";
} while (i < input.length);
return output;
}
//将Base64编码字符串转换成Ansi编码的字符串
function decode64(input) {
var output = "";
var chr1, chr2, chr3 = "";
var enc1, enc2, enc3, enc4 = "";
var i = 0;
 
 
if (input.length % 4 != 0) {
return "";
}
var base64test = /[^A-Za-z0-9\+\/\=]/g;
if (base64test.exec(input)) {
return "";
}
do {
enc1 = keyStr.indexOf(input.charAt(i++));
enc2 = keyStr.indexOf(input.charAt(i++));
enc3 = keyStr.indexOf(input.charAt(i++));
enc4 = keyStr.indexOf(input.charAt(i++));
chr1 = (enc1 << 2) | (enc2 >> 4);
chr2 = ((enc2 & 15) << 4) | (enc3 >> 2);
chr3 = ((enc3 & 3) << 6) | enc4;
 
 
output = output + String.fromCharCode(chr1);
if (enc3 != 64) {
output += String.fromCharCode(chr2);
}
if (enc4 != 64) {
output += String.fromCharCode(chr3);
}
chr1 = chr2 = chr3 = "";
enc1 = enc2 = enc3 = enc4 = "";
} while (i < input.length);
return output;
}
 
 
function utf16to8(str) {
  var out, i, len, c;
 
 
  out = "";
  len = str.length;
  for(i = 0; i < len; i++) {
    c = str.charCodeAt(i);
    if ((c >= 0x0001) && (c <= 0x007F)) {
      out += str.charAt(i);
    } else if (c > 0x07FF) {
      out += String.fromCharCode(0xE0 | ((c >> 12) & 0x0F));
      out += String.fromCharCode(0x80 | ((c >>  6) & 0x3F));
      out += String.fromCharCode(0x80 | ((c >>  0) & 0x3F));
    } else {
      out += String.fromCharCode(0xC0 | ((c >>  6) & 0x1F));
      out += String.fromCharCode(0x80 | ((c >>  0) & 0x3F));
    }
  }
  return out;
}
 
 
function utf8to16(str) {
  var out, i, len, c;
  var char2, char3;
 
 
  out = "";
  len = str.length;
  i = 0;
  while(i < len) {
    c = str.charCodeAt(i++);
    switch(c >> 4) {
      case 0: case 1: case 2: case 3: case 4: case 5: case 6: case 7:
        // 0xxxxxxx
        out += str.charAt(i-1);
        break;
      case 12: case 13:
        // 110x xxxx   10xx xxxx
        char2 = str.charCodeAt(i++);
        out += String.fromCharCode(((c & 0x1F) << 6) | (char2 & 0x3F));
        break;
      case 14:
        // 1110 xxxx  10xx xxxx  10xx xxxx
        char2 = str.charCodeAt(i++);
        char3 = str.charCodeAt(i++);
        out += String.fromCharCode(((c & 0x0F) << 12) |
        ((char2 & 0x3F) << 6) |
        ((char3 & 0x3F) << 0));
        break;
    }
  }
  return out;
}

var testCode= "test88";
var resultCode= encode64(utf16to8(testCode));
console.log(resultCode);

dGVzdDg4

var testCode="dGVzdDg4";
resultCode= utf8to16(decode64(testCode));
console.log(resultCode);

test88