Encode and decode Base64, percent-encoded URLs, and HTML entities, and read the header and payload of a JWT.
Base64 turns bytes into ASCII text that anyone can reverse, since there is no key. See what it is for, the URL-safe variant, padding and the 4/3 size overhead.
Decoding a JWT only reads Base64URL text and does not verify the signature. See the token structure, how to read exp, iat and nbf, and why alg none is rejected.
URLs allow only part of ASCII, so other characters are written as %XX bytes. See why UTF-8 and EUC-KR results differ and when to use encodeURIComponent.