toBinary() and fromBinary() workaround to make atob() and btoa() work with UTF strings. Example for the original blog post at https://base64tool.com website

base64utils.js

/*
* The following snippet is from the Mozilla Developer Portal documentation.
* Utility functions toBinary() and fromBinary() make possible to use btoa() and atob()
* with UTF strings.
*
* The original example here: https://developer.mozilla.org/en-US/docs/Web/API/WindowOrWorkerGlobalScope/btoa
* For additional information see: https://base64tool.com/uncaught-domexception-btoa-on-window/
*
* Usage example:
* > btoa(toBinary('☸☹☺☻☼☾☿'))
* OCY5JjomOyY8Jj4mPyY=
* > fromBinary(atob('OCY5JjomOyY8Jj4mPyY='))
* ☸☹☺☻☼☾☿
**/

// convert a Unicode string to a string in which
// each 16-bit unit occupies only one byte
function toBinary(string) {
  const codeUnits = new Uint16Array(string.length);
  for (let i = 0; i < codeUnits.length; i++) {
    codeUnits[i] = string.charCodeAt(i);
  }
  return String.fromCharCode(...new Uint8Array(codeUnits.buffer));
}

// convert a binary string back into a normal Javascript string
function fromBinary(binary) {
  const bytes = new Uint8Array(binary.length);
  for (let i = 0; i < bytes.length; i++) {
    bytes[i] = binary.charCodeAt(i);
  }
  return String.fromCharCode(...new Uint16Array(bytes.buffer));
}