A fixture sets something up, hands it to the test with use, and tears it down afterward. Test-scoped fixtures (the default) run once per test. Worker-scoped fixtures run once per worker process and are shared by every test that worker runs, which suits expensive setup such as creating an account or opening a connection pool. test.extend takes two type parameters, the test fixtures and then the worker fixtures:

import { test as base } from '@playwright/test';
import { UserPage } from './UserPage';
import { createAccount, deleteAccount, type Account } from './accounts';

type TestFixtures = { userPage: UserPage };
type WorkerFixtures = { account: Account };

export const test = base.extend<TestFixtures, WorkerFixtures>({
  // Once per worker: each parallel worker gets its own account.
  account: [async ({}, use, workerInfo) => {
    const account = await createAccount(`worker-${workerInfo.workerIndex}`);
    await use(account);
    await deleteAccount(account);
  }, { scope: 'worker' }],

  // Once per test: a fresh page object that can use the worker's account.
  userPage: async ({ page, account }, use) => {
    const userPage = new UserPage(page);
    await userPage.signIn(account);
    await use(userPage);
  },
});

A test-scoped fixture can depend on a worker-scoped one, but not the other way round. Add { auto: true } for a fixture every test should get without asking for it, and override a built-in fixture (for example page) the same way you define a new one.

Sources: Playwright fixtures