string-base64-encode-decode.js

//Extend the String object with toBase64() and fromBase64() functions
String.prototype.toBase64 = function () {
  const characters = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/=';
  let result = '';
  let i = 0;
  while (i < this.length) {
    let a = this.charCodeAt(i++) || 0;
    let b = this.charCodeAt(i++) || 0;
    let c = this.charCodeAt(i++) || 0;

    let b1 = (a >> 2) & 0x3F;
    let b2 = ((a & 0x3) << 4) | ((b >> 4) & 0xF);
    let b3 = ((b & 0xF) << 2) | ((c >> 6) & 0x3);
    let b4 = c & 0x3F;

    if (!b) b3 = b4 = 64;
    else if (!c) b4 = 64;
    
    result += characters.charAt(b1) + 
              characters.charAt(b2) + 
              characters.charAt(b3) +
              characters.charAt(b4);
  }
  return result;
}

String.prototype.fromBase64 = function () {
  const characters = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/=';
  let result = '';
  let i = 0;
  while (i < this.length) {
    var b1 = characters.indexOf(this.charAt(i++));
    var b2 = characters.indexOf(this.charAt(i++));
    var b3 = characters.indexOf(this.charAt(i++));
    var b4 = characters.indexOf(this.charAt(i++));

    var a = ((b1 & 0x3F) << 2) | ((b2 >> 4) & 0x3);
    var b = ((b2 & 0xF) << 4) | ((b3 >> 2) & 0xF);
    var c = ((b3 & 0x3) << 6) | (b4 & 0x3F);

    result += String.fromCharCode(a) +
              (b ? String.fromCharCode(b) : '') +
              (c ? String.fromCharCode(c) : '');
  };
  return result;
}