Просмотр кода страницы Java Script - Base64 Encoding/Decoding

 // Create Base64 Object
 var Base64={_keyStr:"ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/=",encode:function(e) {
  var t="";
  var n,r,i,s,o,u,a;
  var f=0;
  e=Base64._utf8_encode(e);
 while(f<e.length){n=e.charCodeAt(f++);
 r=e.charCodeAt(f++);
 i=e.charCodeAt(f++);
 s=n>>2;
 o=(n&3)<<4|r>>4;
 u=(r&15)<<2|i>>6;
 a=i&63;
  if(isNaN(r)){u=a=64}
   else if(isNaN(i)){a=64}t=t+this._keyStr.charAt(s)+this._keyStr.charAt(o)+this._keyStr.charAt(u)+this._keyStr.charAt(a)}
  return t},decode:function(e){
 var t="";
 var n,r,i;
 var s,o,u,a;
 var f=0;
 e=e.replace(/[^A-Za-z0-9\+\/\=]/g,"");
 while(f<e.length {s=this._keyStr.indexOf(e.charAt(f++));
  o=this._keyStr.indexOf(e.charAt(f++));
  u=this._keyStr.indexOf(e.charAt(f++));
  a=this._keyStr.indexOf(e.charAt(f++));
  n=s<<2|o>>4;
  r=(o&15)<<4|u>>2;
  i=(u&3)<<6|a;
  t=t+String.fromCharCode(n);
 if(u!=64){t=t+String.fromCharCode(r)}
 if(a!=64){t=t+String.fromCharCode(i)}
  }t=Base64._utf8_decode(t);
 return t},_utf8_encode:function(e){e=e.replace(/\r\n/g,"\n");
 var t="";
 for(var n=0; n<e.length; n++){
 var r=e.charCodeAt(n);
 if(r<128){
 t+=String.fromCharCode(r)}else if(r>127&&r<2048){t+=String.fromCharCode(r>>6|192);
 t+=String.fromCharCode(r&63|128)}
  else{t+=String.fromCharCode(r>>12|224);
 t+=String.fromCharCode(r>>6&63|128);
 t+=String.fromCharCode(r&63|128)}}return t},_utf8_decode:function(e){
 var t="";
 var n=0;
 var r=c1=c2=0;
 while(n<e.length){r=e.charCodeAt(n);
 if(r<128){t+=String.fromCharCode(r);
 n++}
  else if(r>191&&r<224){c2=e.charCodeAt(n+1);
 t+=String.fromCharCode((r&31)<<6|c2&63);
 n+=2}
  else{c2=e.charCodeAt(n+1);
 c3=e.charCodeAt(n+2);
 t+=String.fromCharCode((r&15)<<12|(c2&63)<<6|c3&63);
 n+=3}}return t}} 

 // Define the string
 var string = 'Hello World!';

 // Encode the String
 var encodedString = Base64.encode(string);
 console.log(encodedString); // Outputs: "SGVsbG8gV29ybGQh"

 // Decode the String
 var decodedString = Base64.decode(encodedString);
 console.log(decodedString); // Outputs: "Hello World!"

[https://gist.github.com/ncerminara/11257943#file-base64-js взято тут]