base64 encoder (btoa) / decoder (aotb) shim for NativeScript

base64.ts

// base64 encoder (btoa) / decoder (atob) which you can use in your NativeScript app.
// 
// Usage (assuming you're in some component and this file is in the same folder:
//
//   require('./base64');
//
//   const username = "My Usernamé";
//   usernameBase64Encoded = btoa(username);


(function () {
  // only works in {N}
  if (typeof global === "undefined") {
    return;
  }

  var chars = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/=';

  function InvalidCharacterError(message) {
    this.message = message;
  }
  InvalidCharacterError.prototype = new Error;
  InvalidCharacterError.prototype.name = 'InvalidCharacterError';

  // encoder
  // [https://gist.github.com/999166] by [https://github.com/nignag]
  global['btoa'] = function (input) {
    var str = String(input);
    for (
      // initialize result and counter
      var block, charCode, idx = 0, map = chars, output = '';
      // if the next str index does not exist:
      //   change the mapping table to "="
      //   check if d has no fractional digits
      str.charAt(idx | 0) || (map = '=', idx % 1);
      // "8 - idx % 1 * 8" generates the sequence 2, 4, 6, 8
      output += map.charAt(63 & block >> 8 - idx % 1 * 8)
    ) {
      charCode = str.charCodeAt(idx += 3/4);
      if (charCode > 0xFF) {
        throw new InvalidCharacterError("'btoa' failed: The string to be encoded contains characters outside of the Latin1 range.");
      }
      block = block << 8 | charCode;
    }
    return output;
  };

  // decoder
  // [https://gist.github.com/1020396] by [https://github.com/atk]
  global['atob'] = function (input) {
    var str = String(input).replace(/=+$/, '');
    if (str.length % 4 === 1) {
      throw new InvalidCharacterError("'atob' failed: The string to be decoded is not correctly encoded.");
    }
    for (
      // initialize result and counters
      var bc = 0, bs, buffer, idx = 0, output = '';
      // get next character
      buffer = str.charAt(idx++);
      // character found in table? initialize bit storage and add its ascii value;
      ~buffer && (bs = bc % 4 ? bs * 64 + buffer : buffer,
        // and if not first of each 4 characters,
        // convert the first 8 bits to one ascii character
      bc++ % 4) ? output += String.fromCharCode(255 & bs >> (-2 * bc & 6)) : 0
    ) {
      // try to find character in table (0-63, not found => -1)
      buffer = chars.indexOf(buffer);
    }
    return output;
  };
}());