UUID generator

Which version to use

Version 4 is 122 random bits and nothing else: unguessable, unordered, and a poor database key at scale, because consecutive inserts land all over a B-tree index.

Version 7 puts a 48-bit big-endian millisecond timestamp in the leading bytes (RFC 9562 §5.7), so a plain lexicographic or byte-wise sort is also a sort by creation time — that is the entire reason to choose it. Ids generated inside the same millisecond stay in order too, via a 12-bit counter in the following bits, so a batch of a thousand is still ordered. The timestamp is visible to anyone holding the id, so v7 is not a choice for anything that must not leak when it was created.

Both come from crypto.getRandomValues (v4 prefers crypto.randomUUID where the browser provides it) inside this tab. Nothing is sent anywhere and nothing is stored, so reloading the page loses them.