Random UUIDs with uuid v4

A version 4 UUID is built from random bytes and is the usual choice for ids of records, requests and keys.

How it works

Call v4(). Import v4 (renamed uuidv4) and call it with no arguments. Each call returns a new UUID string, used here as an id for each todo. [1]
Random source. The package fills the random bytes from the platform crypto API. The rng option lets you pass your own function; here it calls crypto.getRandomValues directly. [1]
Fixed bytes for tests. Passing random with 16 fixed bytes gives the same UUID every run, which is handy in tests. getRandomValues is the browser call that fills a typed array with random numbers. [2]

Run and edit it

It runs in your browser, in a sandboxed frame that cannot see this site; the packages load from esm.sh only after you press Run.

import { v4 as uuidv4 } from 'uuid'

// Each call returns a new random UUID string
console.log('v4 #1:', uuidv4())
console.log('v4 #2:', uuidv4())
console.log('v4 #3:', uuidv4())

// Give every record its own id
const todos = ['Write docs', 'Ship release', 'Reply to issues'].map((title) => ({ id: uuidv4(), title }))
for (const t of todos) console.log(`todo "${t.title}" ->`, t.id)

// rng option: a function that returns 16 random bytes
const rng = () => crypto.getRandomValues(new Uint8Array(16))
console.log('with rng option:', uuidv4({ rng }))

// random option: fixed bytes always give the same UUID
const random = Uint8Array.of(
  0x10, 0x91, 0x56, 0xbe, 0xc4, 0xfb, 0xc1, 0xea,
  0x71, 0xb4, 0xef, 0xe1, 0x67, 0x1c, 0x58, 0x36
)
console.log('with fixed random bytes:', uuidv4({ random }))
console.log('string length:', uuidv4().length)

Ctrl/⌘ + Enter runs

Run it a few times: the plain v4() lines change, the fixed-bytes line stays 109156be-c4fb-41ea-b1b4-efe1671c5836. Code written by AgentGid [1].

Checked Oct 12, 2026: it starts without errors in Chromium with uuid 14.0.3. Needs WebGL.

More uuid examples

Time-based UUIDs with uuid v7 Name-based UUIDs with uuid v5 Validating UUIDs with uuid Converting UUIDs to bytes with uuid

Sources

Explanations written by AgentGid from the official docs; the numbers in brackets point here. The code marked "written by AgentGid" is ours and free to copy.

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