JavaScript实现Base64编码

编码规则

Base64编码的思想是是采用64个基本的ASCII码字符对数据进行重新编码。它将需要编码的数据拆分成字节数组。以3个字节为一组。按顺序排列24 位数据，再把这24位数据分成4组，即每组6位。再在每组的的最高位前补两个0凑足一个字节。这样就把一个3字节为一组的数据重新编码成了4个字节。当所要编码的数据的字节数不是3的整倍数，也就是说在分组时最后一组不够3个字节。这时在最后一组填充1到2个0字节。并在最后编码完成后在结尾添加1到2个 “=”。

代码实现

  //下面是64个基本的编码
  var base64EncodeChars = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";
  var base64DecodeChars = new Array(
      -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
      -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
      -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, 62, -1, -1, -1, 63,
      52, 53, 54, 55, 56, 57, 58, 59, 60, 61, -1, -1, -1, -1, -1, -1,
      -1,  0,  1,  2,  3,  4,  5,  6,  7,  8,  9, 10, 11, 12, 13, 14,
      15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, -1, -1, -1, -1, -1,
     -1, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40,
     41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, -1, -1, -1, -1, -1);
 //编码的方法
 function base64encode(str) {
     var out, i, len;
     var c1, c2, c3;
     len = str.length;
     i = 0;
     out = "";
     while(i < len) {
     c1 = str.charCodeAt(i++) & 0xff;
     if(i == len)
     {
         out += base64EncodeChars.charAt(c1 >> 2);
         out += base64EncodeChars.charAt((c1 & 0x3) << 4);
         out += "==";
         break;
     }
     c2 = str.charCodeAt(i++);
     if(i == len)
     {
         out += base64EncodeChars.charAt(c1 >> 2);
         out += base64EncodeChars.charAt(((c1 & 0x3)<< 4) | ((c2 & 0xF0) >> 4));
         out += base64EncodeChars.charAt((c2 & 0xF) << 2);
         out += "=";
         break;
     }
     c3 = str.charCodeAt(i++);
     out += base64EncodeChars.charAt(c1 >> 2);
     out += base64EncodeChars.charAt(((c1 & 0x3)<< 4) | ((c2 & 0xF0) >> 4));
     out += base64EncodeChars.charAt(((c2 & 0xF) << 2) | ((c3 & 0xC0) >>6));
     out += base64EncodeChars.charAt(c3 & 0x3F);
     }
     return out;
 }
 //解码的方法
 function base64decode(str) {
     var c1, c2, c3, c4;
     var i, len, out;
     len = str.length;
     i = 0;
     out = "";
     while(i < len) {

     do {
         c1 = base64DecodeChars[str.charCodeAt(i++) & 0xff];
     } while(i < len && c1 == -1);
     if(c1 == -1)
         break;

     do {
         c2 = base64DecodeChars[str.charCodeAt(i++) & 0xff];
     } while(i < len && c2 == -1);
     if(c2 == -1)
         break;
     out += String.fromCharCode((c1 << 2) | ((c2 & 0x30) >> 4));

     do {
         c3 = str.charCodeAt(i++) & 0xff;
         if(c3 == 61)
         return out;
         c3 = base64DecodeChars[c3];
     } while(i < len && c3 == -1);
     if(c3 == -1)
         break;
     out += String.fromCharCode(((c2 & 0XF) << 4) | ((c3 & 0x3C) >> 2));

     do {
         c4 = str.charCodeAt(i++) & 0xff;
         if(c4 == 61)
         return out;
         c4 = base64DecodeChars[c4];
     } while(i < len && c4 == -1);
     if(c4 == -1)
         break;
     out += String.fromCharCode(((c3 & 0x03) << 6) | c4);
     }
     return out;
 }
 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  case  case  case  case  case  case  case
        // 0xxxxxxx
        out += str.charAt(i-1);
        break;
      case  case
        // 110x xxxx   10xx xxxx
        char2 = str.charCodeAt(i++);
        out += String.fromCharCode(((c & 0x1F) << 6) | (char2 & 0x3F));
        break;
      case
        // 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;
}

调用

//编码
value = base64encode(utf16to8(src))

//解码
value = utf8to16(base64decode(src))