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Seeded Test Fixtures for Vitest ​

Problem ​

Test suites need realistic mock data that stays stable across runs so snapshots and assertions do not flake in CI. Hand-written fixtures drift from real shapes; unseeded random generators produce different output every run.

APIs involved: createIllusion, person, internet, location, system.

Solution ​

Create a seeded illusionist instance inside each test. The same seed reproduces the same data in every run.

ts
import { createIllusion } from '@vielzeug/illusionist';
import { en } from '@vielzeug/illusionist/locales';
import { describe, expect, test } from 'vitest';

function makeUser(seed: number) {
  const illusion = createIllusion({ seed, locale: en });

  const user = {
    name: illusion.person.fullName(),
    email: illusion.internet.email(),
    address: illusion.location.streetAddress(),
    city: illusion.location.city(),
    zip: illusion.location.zipCode(),
  };

  illusion.dispose();
  return user;
}

describe('user profile', () => {
  test('generates a valid user', () => {
    const user = makeUser(12345);

    expect(user.name).toContain(' ');
    expect(user.email).toContain('@');
  });

  test('is deterministic for the same seed', () => {
    const a = makeUser(12345);
    const b = makeUser(12345);

    expect(a).toEqual(b);
  });

  test('snapshot stays stable', () => {
    const user = makeUser(12345);

    expect(user).toMatchInlineSnapshot();
  });
});

With per-test isolation ​

Give each test a distinct seed so failures point to a specific case. Use using to release the instance automatically.

ts
import { createIllusion } from '@vielzeug/illusionist';
import { en } from '@vielzeug/illusionist/locales';
import { describe, expect, test } from 'vitest';

describe('orders', () => {
  test.each([1, 2, 3])('order #%s', (seed) => {
    using illusion = createIllusion({ seed, locale: en });

    const order = {
      customer: illusion.person.fullName(),
      product: illusion.commerce.productName(),
      price: illusion.commerce.price({ min: 10, max: 100 }),
    };

    expect(order.price.amount).toBeGreaterThan(0n);
  });
});

Pitfalls ​

  • Do not share a single instance across tests that run concurrently — each call advances the shared random source and causes cross-test drift.
  • Do not reuse an instance after dispose(); create a fresh one per test instead.
  • String seeds are hashed, so two different strings never collide, but the same string always produces the same sequence.
  • system.uuid() uses crypto.randomUUID(), not the seeded source. Do not use it in deterministic fixtures or snapshot tests.