# uuidv7: A JavaScript implementation of UUID version 7

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```javascript
import { uuidv7 } from "uuidv7";

const result = uuidv7(); // e.g., "017fe537-bb13-7c35-b52a-cb5490cce7be"
```

On browsers and Deno:

```javascript
import { uuidv7 } from "https://unpkg.com/uuidv7@^1";

const result = uuidv7(); // e.g., "017fe537-bb13-7c35-b52a-cb5490cce7be"
```

Command-line interface:

```bash
$ npx uuidv7
0189f7e5-c883-7106-8272-ccb7fcba0575
$
$ npx uuidv7 -n 4
0189f7ea-ae2c-7809-8aeb-b819cf5e9e7f
0189f7ea-ae2f-72b9-9be8-9c3c5a60214f
0189f7ea-ae2f-72b9-9be8-9c3d224082ef
0189f7ea-ae2f-72b9-9be8-9c3e3e8abae8
```

See [RFC 9562](https://www.rfc-editor.org/rfc/rfc9562).

## Field and bit layout

This implementation produces identifiers with the following bit layout:

```text
 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                          unix_ts_ms                           |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|          unix_ts_ms           |  ver  |        counter        |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|var|                        counter                            |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                             rand                              |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
```

Where:

- The 48-bit `unix_ts_ms` field is dedicated to the Unix timestamp in
  milliseconds.
- The 4-bit `ver` field is set at `0111`.
- The 42-bit `counter` field accommodates a counter that ensures the increasing
  order of IDs generated within a millisecond. The counter is incremented by one
  for each new ID and is reset to a random number when the `unix_ts_ms` changes.
- The 2-bit `var` field is set at `10`.
- The remaining 32 `rand` bits are filled with a cryptographically strong random
  number.

The 42-bit `counter` is sufficiently large, so you do not usually need to worry
about overflow, but in an extremely rare circumstance where it overflows, this
library increments the `unix_ts_ms` field to continue instant monotonic
generation. As a result, the `unix_ts_ms` may have a greater value than that of
the system's real-time clock. (See also [Why so large counter? (42bits)]).

UUIDv7, by design, relies on the system clock to guarantee the monotonically
increasing order of generated IDs. A generator may not be able to produce a
monotonic sequence if the system clock goes backwards. This library ignores a
clock rollback and reuses the previous `unix_ts_ms` unless the clock rollback is
considered significant (by default, more than ten seconds). If such a
significant rollback takes place, this library resets the generator by default
and thus breaks the increasing order of generated IDs.

[Why so large counter? (42bits)]: https://github.com/LiosK/uuidv7/issues/13#issuecomment-2306922356

## Other features

This library also supports the generation of UUID version 4:

```javascript
import { uuidv4 } from "uuidv7";

const result = uuidv4(); // e.g., "83229083-75c3-4da5-8378-f88ef1a2bcd1"
```

`uuidv7obj()` and `uuidv4obj()` return an object that represents a UUID as a
16-byte byte array:

```javascript
import { uuidv7obj } from "uuidv7";

const object = uuidv7obj();
console.log(object.bytes); // Uint8Array(16) [ ... ]
console.log(String(object)); // e.g., "017fea6b-b877-7aef-b422-57db9ed15e9d"

console.assert(object.getVariant() === "VAR_10");
console.assert(object.getVersion() === 7);

console.assert(object.clone().equals(object));
console.assert(object.compareTo(uuidv7obj()) < 0);
```

The `V7Generator` primitive allows to utilize a separate counter state from that
of the global generator. It also provides a fallible variant of the generator
function to give an absolute guarantee of the increasing order of UUIDs despite
a significant rollback of the system timestamp source.

```javascript
import { V7Generator } from "uuidv7";

const g = new V7Generator();
const x = g.generate();
const y = g.generateOrAbort();
if (y === undefined) {
  throw new Error("The clock went backwards by ten seconds!");
}
console.assert(x.compareTo(y) < 0);
```

See the [API documentation](https://liosk.github.io/uuidv7/) for details.

## CommonJS support

The CommonJS entry point is deprecated and provided for backward compatibility
purposes only. The entry point is no longer tested and will be removed in the
future.

## License

Licensed under the Apache License, Version 2.0.

## Related project

[Uuid25](https://www.npmjs.com/package/uuid25) provides the conversion to/from a
condensed, sortable, case-insensitive, 25-digit Base36 representation of UUID as
well as other well-known textual representations. Uuid25 is available in several
languages including Go, JavaScript, Python, Rust, and Swift.

```javascript
import { uuidv7obj } from "uuidv7";
import { Uuid25 } from "uuid25";

const uuid25 = Uuid25.fromBytes(uuidv7obj().bytes);

console.log(uuid25.value);
// e.g., "03a2s63x4x0b9mev9e88i7gpm"

console.log(uuid25.toHex());
// e.g., "0189f8068f1a79b6bb21123c6accc25a"
console.log(uuid25.toHyphenated());
// e.g., "0189f806-8f1a-79b6-bb21-123c6accc25a"
console.log(uuid25.toBraced());
// e.g., "{0189f806-8f1a-79b6-bb21-123c6accc25a}"
console.log(uuid25.toUrn());
// e.g., "urn:uuid:0189f806-8f1a-79b6-bb21-123c6accc25a"
```