Please include base-64 utilities

Adobe XD UXP Plugin API
11 
function strToUTF8Base64(str) {
  function decodeSurrogatePair(hi, lo) {
    var resultChar = 0x010000;
    resultChar += lo - 0xdc00;
    resultChar += (hi - 0xd800) << 10;
    return resultChar;
  }

  var bytes = [0, 0, 0];
  var byteIndex = 0;
  var result = [];

  function output(s) {
    result.push(s);
  }

  function emitBase64() {
    var digits =
      "ABCDEFGHIJKLMNOPQRSTUVWXYZ" +
      "abcdefghijklmnopqrstuvwxyz" +
      "0123456789+/";

    function toDigit(value) {
      return digits[value];
    }

    // --Byte 0--    --Byte 1--    --Byte 2--
    // 1111  1122    2222  3333    3344  4444

    var d1 = toDigit(bytes[0] >> 2);
    var d2 = toDigit(((bytes[0] & 0x03) << 4) | (bytes[1] >> 4));
    var d3 = toDigit(((bytes[1] & 0x0f) << 2) | (bytes[2] >> 6));
    var d4 = toDigit(bytes[2] & 0x3f);

    if (byteIndex === 1) {
      output(d1 + d2 + "==");
    } else if (byteIndex === 2) {
      output(d1 + d2 + d3 + "=");
    } else {
      output(d1 + d2 + d3 + d4);
    }
  }

  function emit(chr) {
    bytes[byteIndex++] = chr;
    if (byteIndex == 3) {
      emitBase64();
      bytes[0] = 0;
      bytes[1] = 0;
      bytes[2] = 0;
      byteIndex = 0;
    }
  }

  function emitLast() {
    if (byteIndex > 0) {
      emitBase64();
    }
  }

  // Converts the string to UTF8:

  var i, chr;
  var hi, lo;
  for (i = 0; i < str.length; i++) {
    chr = str.charCodeAt(i);

    // Test and decode surrogate pairs in the string
    if (chr >= 0xd800 && chr <= 0xdbff) {
      hi = chr;
      lo = str.charCodeAt(i + 1);
      if (lo >= 0xdc00 && lo <= 0xdfff) {
        chr = decodeSurrogatePair(hi, lo);
        i++;
      }
    }

    // Encode the character as UTF-8.
    if (chr < 0x80) {
      emit(chr);
    } else if (chr < 0x0800) {
      emit((chr >> 6) | 0xc0);
      emit(((chr >> 0) & 0x3f) | 0x80);
    } else if (chr < 0x10000) {
      emit((chr >> 12) | 0xe0);
      emit(((chr >> 6) & 0x3f) | 0x80);
      emit(((chr >> 0) & 0x3f) | 0x80);
    } else if (chr < 0x110000) {
      emit((chr >> 18) | 0xf0);
      emit(((chr >> 12) & 0x3f) | 0x80);
      emit(((chr >> 6) & 0x3f) | 0x80);
      emit(((chr >> 0) & 0x3f) | 0x80);
    }
  }

  emitLast();

  return result.join("");
}

Taken from this site https://stackoverflow.com/questions/246801/how-can-you-encode-a-string-to-base64-in-javascript

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