testing $state, $derived, and $effect.root does not behave as expected

[win-chen]

Describe the bug

Hello, in my attempt to recreate the tests in https://svelte.dev/docs/svelte/testing, I've run into test cases that do not behave as expected. Here is the test file with two tests "runes" and "effect.root":

import { flushSync } from "svelte";
import { describe, expect, it } from "vitest";

describe("svelte", () => {
  it("runes", () => {
    let state = $state("hello");
    let der = $derived(state.length);

    state = "bye";
    flushSync();

    expect(der).toBe(3);
  });

  it("effect.root", () => {
    const cleanup = $effect.root(() => {
      expect(true).toBe(false);
    });
    cleanup();
  });
});

1. "runes"
   Expected behavior: passes with der = 3.
   Actual behavior: fails with der = 5 instead of 3.

2. "effect.root"
   Expected behavior: fails due to true !== false
   Actual behavior: fails with "expected any number of assertion, but got none"

I believe the tests are configured correctly because non-svelte tests using $state would fail on $state not found.
Please lmk if this is actually configuration error or if expectations are incorrect.
Tests ran with svelte version 5.40.0, 5.41.0, and 5.43.0 - same behaviors.

Reproduction

Repo with the reproduction:
https://github.com/win-chen/svelte-test-bug

Run tests:

npm install
npm run test

View "src/test.svelte.spec.ts" for the tests in question. There is a third test in this file which is just the copy of the "Effect" test in the docs https://svelte.dev/docs/svelte/testing

Logs

System Info

System:
    OS: macOS 14.6.1
    CPU: (8) arm64 Apple M2
    Memory: 79.09 MB / 16.00 GB
    Shell: 5.9 - /bin/zsh
  Binaries:
    Node: 22.14.0 - /Users/winchen/.nvm/versions/node/v22.14.0/bin/node
    Yarn: 1.22.22 - /Users/winchen/.nvm/versions/node/v22.14.0/bin/yarn
    npm: 10.9.2 - /Users/winchen/.nvm/versions/node/v22.14.0/bin/npm
    pnpm: 10.15.0 - /Users/winchen/.nvm/versions/node/v22.14.0/bin/pnpm
  Browsers:
    Chrome: 142.0.7444.135
    Safari: 17.6
  npmPackages:
    svelte: ^5.41.0 => 5.43.6

Severity

annoyance

[7nik]

environment: "node",

Your setup tests runes in the server environment, where reactivity does not happen - thus $derived does not update, and all $effects and $effect.root are not executed.

Set up Vitest according to the docs.