Question 1

Which UUID version should I use?

Accepted Answer

v4 unless you have a reason not to, and v7 when the UUID will be a primary key or anything else that gets indexed or sorted. v5 when the id has to be reproducible from some existing name, so two systems can derive the same id without talking to each other. v1 and v6 only when you are joining a system that already uses them, and v3 only when something already depends on MD5.

Question 2

Are these UUIDs safe to use in production?

Accepted Answer

Yes. Version 4 comes from crypto.randomUUID where the browser provides it, which is the same generator a server-side library would reach for, and everything else is built on crypto.getRandomValues. Both are cryptographically secure sources rather than Math.random. The generation happens on your device, so nothing is sent anywhere and no two visitors can be handed the same value.

Question 3

Can two UUIDs ever be the same?

Accepted Answer

In theory yes, in practice no, for v4. There are 122 random bits, so you would need roughly 2.7 quintillion of them before reaching a 50 percent chance of a single collision, which is more than any system will generate. What does cause duplicates in the real world is a broken generator: seeding from a clock, using Math.random, or copying a hardcoded UUID from a tutorial.

Question 4

Why do the v7 UUIDs I generate in one batch still sort correctly?

Accepted Answer

Because a batch is created inside the same millisecond, and a plain timestamp would give them all identical leading bits and random order after that. RFC 9562 section 6.2 allows a counter in the 12 bits following the timestamp, and this generator uses it, so a batch of a hundred comes out strictly increasing rather than merely grouped by millisecond.

Question 5

Does a v1 UUID expose my MAC address?

Accepted Answer

Not from here. The original specification put the network card's MAC address in the last 48 bits, which is how a v1 UUID in a Word document helped identify the author of the Melissa virus in 1999. A browser cannot read a MAC address at all, so this generator uses random bits with the multicast bit set, exactly as RFC 9562 section 6.10 prescribes, which guarantees the value can never be mistaken for real hardware.

Question 6

What are the nil and max UUIDs for?

Accepted Answer

The nil UUID is all zeros and stands for the absence of a UUID, which is useful in a column that cannot be null or as an explicit "unset" marker. The max UUID is all ones and was added in RFC 9562 as the other bookend: since it sorts after every possible UUID, it makes a clean upper bound for a range query.

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