js_base64_url_safe.js

let text = "поздравляю со сдачей сессии"
let encoded = btoa(text)
let decoded = atob(encoded)

function utf8_to_b64( str ) {
  return window.btoa(unescape(encodeURIComponent( str )));
}

function b64_to_utf8( str ) {
  return decodeURIComponent(escape(window.atob( str )));
}

encoded = utf8_to_b64(text)

let urlSafeEncoded = encoded.replaceAll('/', '_').replaceAll('+', '-')
let urlSafeDecoded = urlSafeEncoded.replaceAll('_', '/').replaceAll('-', '+')

decoded = b64_to_utf8(urlSafeDecoded)