Most of us, when generating a UUID, will reach for a library to do it. Even a UUIDv4, which is just a random number, presents challenges: doing randomness correctly is hard, and certain bits within the UUID are reserved for metadata about what kind of UUID we’re generating.
But Gretchen‘s co-worker didn’t reach for a library. What they did reach for was… regular expressions?
function uuidv4() {
return "xxxxxxxx-xxxx-4xxx-yxxx-xxxxxxxxxxxx".replace(/[xy]/g, function (c) {
var r = (Math.random() * 16) | 0,
v = c == "x" ? r : (r & 0x3) | 0x8;
return v.toString(16);
});
}
At a glance, this appears to be a riff on common answers on Stack Overflow. I won’t pick on this code for not using crypto.randomUUID, the browser function for doing this, as that function only started showing up in browsers in 2021. But using a format string and filling it with random data instead of generating your 128-bits as a Uint8Buffer is less forgivable.
This solution to generating UUIDs makes a common mistake: confusing the representation of the data with the reality of the data. A UUID is 128-bits of numerical data, with a few bits reserved for identification (annoyingly, how many bits are reserved depends on which format we’re talking about). We render it as the dash-separated-hex-string, but it is not a dash-separated-hex-string.
In the end, this code does work. Awkwardly and inefficiently and with a high probability of collisions due to bad randomness, but it works. I just hate it.
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