js解析base64

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 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;
  }

   //编码
   value = base64encode(utf16to8(src))
      
  //解码
  value = utf8to16(base64decode(src))