function Base64() {

// private property 
_keyStr = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/="; 

// public method for encoding 
this.encode = function (input) { 
 var output = ""; 
 var chr1, chr2, chr3, enc1, enc2, enc3, enc4; 
 var i = 0; 
 input = _utf8_encode(input); 
 while (i < input.length) { 
  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); 
 } 
 return output; 
} 

// public method for decoding 
this.decode = function (input) { 
 var output = ""; 
 var chr1, chr2, chr3; 
 var enc1, enc2, enc3, enc4; 
 var i = 0; 
 input = input.replace(/[^A-Za-z0-9\+\/\=]/g, ""); 
 while (i < input.length) { 
  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 = output + String.fromCharCode(chr2); 
  } 
  if (enc4 != 64) { 
   output = output + String.fromCharCode(chr3); 
  } 
 } 
 output = _utf8_decode(output); 
 return output; 
} 

// private method for UTF-8 encoding 
_utf8_encode = function (string) { 
 string = string.replace(/\r\n/g,"\n"); 
 var utftext = ""; 
 for (var n = 0; n < string.length; n++) { 
  var c = string.charCodeAt(n); 
  if (c < 128) { 
   utftext += String.fromCharCode(c); 
  } else if((c > 127) && (c < 2048)) { 
   utftext += String.fromCharCode((c >> 6) | 192); 
   utftext += String.fromCharCode((c & 63) | 128); 
  } else { 
   utftext += String.fromCharCode((c >> 12) | 224); 
   utftext += String.fromCharCode(((c >> 6) & 63) | 128); 
   utftext += String.fromCharCode((c & 63) | 128); 
  } 

 } 
 return utftext; 
} 

// private method for UTF-8 decoding 
_utf8_decode = function (utftext) { 
 var string = ""; 
 var i = 0; 
 var c = c1 = c2 = 0; 
 while ( i < utftext.length ) { 
  c = utftext.charCodeAt(i); 
  if (c < 128) { 
   string += String.fromCharCode(c); 
   i++; 
  } else if((c > 191) && (c < 224)) { 
   c2 = utftext.charCodeAt(i+1); 
   string += String.fromCharCode(((c & 31) << 6) | (c2 & 63)); 
   i += 2; 
  } else { 
   c2 = utftext.charCodeAt(i+1); 
   c3 = utftext.charCodeAt(i+2); 
   string += String.fromCharCode(((c & 15) << 12) | ((c2 & 63) << 6) | (c3 & 63)); 
   i += 3; 
  } 
 } 
 return string; 
} 
 

}
用例例子