Svelte

vitest-auto-spy/svelte turns a class-based service or store into a typed spy. A spy is a stand-in object: every method records its calls and returns what you set. You give the spy to the component the same way it gets the real object: through props, context, or the module it imports.

Through props

import { fireEvent, render, screen } from '@testing-library/svelte';
import { expect, it } from 'vitest';
import { createSpyFromClass } from 'vitest-auto-spy/svelte';

import Cart from './Cart.svelte';
import { CartStore } from './cart-store';

it('renders the total the store reports', () => {
  const cartStore = createSpyFromClass(CartStore);

  cartStore.total.mockReturnValue(42);

  render(Cart, { props: { store: cartStore } });

  expect(screen.getByText('$42')).toBeInTheDocument();
  expect(cartStore.total).toHaveBeenCalled();
});

it('checks out when the button is clicked', async () => {
  const cartStore = createSpyFromClass(CartStore);

  cartStore.checkout.resolveWith({ orderId: 'ord_42' }); // checkout() returns a Promise

  render(Cart, { props: { store: cartStore } });
  await fireEvent.click(screen.getByRole('button', { name: 'Check out' }));

  expect(cartStore.checkout).toHaveBeenCalledTimes(1);
});

You set answers with mockReturnValue (a plain value) or resolveWith (a resolved promise), and check calls with the usual toHaveBeenCalled… matchers. toBeInTheDocument needs import '@testing-library/jest-dom/vitest' in your Vitest setup file.

vitest-auto-spy/svelte is an entry: the import path you use in Svelte specs. It exports the core API of the library: createSpyFromClass, createAutoMock, the Spy<T> type, asInstance, resetAutoSpy and the rest. Svelte has no class-based dependency injection, so there are no extra helpers.

Through context

If the component reads the store with getContext, pass the spy under the same key:

import { render, screen } from '@testing-library/svelte';
import { expect, it } from 'vitest';
import { createSpyFromClass } from 'vitest-auto-spy/svelte';

import Cart from './Cart.svelte';
import { CART_STORE, CartStore } from './cart-store';

it('renders the total the store reports', () => {
  const cartStore = createSpyFromClass(CartStore);

  cartStore.total.mockReturnValue(42);

  render(Cart, { context: new Map([[CART_STORE, cartStore]]) });

  expect(screen.getByText('$42')).toBeInTheDocument();
});

Through a mocked module

If the component imports a ready instance (a singleton) from a module, replace that export with the spy:

import { render } from '@testing-library/svelte';
import { expect, it, vi } from 'vitest';
import { createSpyFromClass } from 'vitest-auto-spy/svelte';

import { CartStore } from './cart-store';

const cartStore = vi.hoisted(() => createSpyFromClass(CartStore));

vi.mock('./cart-store', async (importOriginal) => ({
  ...(await importOriginal<typeof import('./cart-store')>()),
  cartStore,
}));

it('checks out through the module singleton', async () => {
  cartStore.checkout.resolveWith({ orderId: 'ord_42' });

  render(await import('./Cart.svelte').then((m) => m.default));

  expect(cartStore.checkout).toHaveBeenCalled();
});

Common mistake: creating the spy in a plain const at the top of the file. Vitest moves vi.mock above all other code, so the factory cannot see that variable. Wrap it in vi.hoisted, as above.

Which runner

This entry never imports svelte or @testing-library/svelte. It does import vitest, so it does not load under bun test or node --test. On those runners, import createSpyFromClass from your runner's entry (vitest-auto-spy/bun, vitest-auto-spy/node).

If a setup file already imports a runner entry such as vitest-auto-spy/bun, that runner's mocks stay in use after you import this entry.

In depth

When vi.mock has nothing to replace

vi.mock replaces a module by its path. Some test setups bundle the spec together with its imports into one file (for example, the @angular/build:unit-test builder), so a relative path no longer points to a separate module. Then vi.mock changes nothing and the component uses the real store. Pass the spy through props or context instead.