Base62 commonly uses digits and upper- and lowercase letters. That makes it attractive for compact identifiers with no punctuation. Base64 uses 64 symbols and packs groups of bits directly. Although the names are similar, Base62 implementations often model the input as a number, which creates different interoperability questions.
The alphabet order is part of the format
Two Base62 libraries can choose different orders for digits, uppercase letters and lowercase letters. The same visible string then represents different numbers. A system using 0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz does not agree with one that places lowercase letters first. Record the alphabet and test vectors in the protocol; “Base62” alone is not enough.
Case sensitivity matters too. A lowercasing proxy, a case-insensitive database collation or a manually normalized identifier can change its value. If your storage layer ignores case, neither ordinary Base62 nor Base64 is a safe representation without an additional design decision.
Preserve length when encoding bytes
Consider two files containing hexadecimal 01 and 0001. Interpreted as unsigned integers, both have value one. A bare integer-to-Base62 conversion loses the distinction. A byte-oriented Base62 format needs explicit leading-zero handling or a length field. This is especially relevant for random tokens and fixed-length cryptographic values.
const a = BigInt('0x01');
const b = BigInt('0x0001');
console.log(a === b); // true: numeric equality loses byte lengthChoose based on what the value means
If you are shortening a database integer under one application's control, a documented Base62 alphabet can be convenient. If you are transporting arbitrary bytes between independent systems, Base64URL usually provides a clearer standardized contract. It substitutes dash and underscore for the standard Base64 punctuation while retaining the same six-bit grouping.
Encoding is not randomization. Sequential integers remain predictable after Base62 conversion. If identifiers need to resist guessing, generate sufficient unpredictable entropy using a cryptographic random source, then encode the resulting bytes without losing length. Do not treat a compact-looking token as proof that its generation is secure.
When comparing output sizes, include prefixes, padding rules and any length metadata. Roundtrip at least an empty input, a zero byte, several leading zeros and a high-valued byte. The Base64URL alphabet is standardized in RFC 4648; a Base62 application must specify the additional details itself.