`$effect` not working in tests, but `$derived` does

[rChaoz]

Describe the bug

I'm having this weird issue where $effect is not working in tests. Interestingly, $derived does work, which means Svelte is not compiling in SSR mode. For example, in this test:

let num = $state(0)
let doubled = $derived(num * 2)
num = 5
expect(doubled).toBe(10)

$effect.root(() => {
    let a = 0
    $effect(() => {
        a = 5
    })
    flushSync()
    expect(a).toBe(5)
})

it errors at the very end: expected +0 to be 5, but it should work. Here's my Vite config:

export default defineConfig({
    plugins: [
        enhancedImages(),
        sveltekit(),
        ...
    ],
    test: {
        include: ["src/**/*.test.{js,ts}"],
        coverage: { ... },
        environmentMatchGlobs: [["src/**/*.svelte.test.{js,ts}", "jsdom"]],
        setupFiles: "src/vitest.setup.ts",
    },
    build: { ... },
})

I also have this mock in my global setup as I use if (browser) in other places in my tests:

vi.mock(import("esm-env"), async (importOriginal): Promise<typeof import("esm-env")> => {
    const module = await importOriginal()
    if (typeof window == "object")
        return {
            ...module,
            BROWSER: true,
        }
    else return module
})

I tried removing it, but without success. I'm not quite sure how to reproduce this yet. Is there something obviously wrong with my config, or something I can try?

[7nik]

$derived does work, which means Svelte is not compiling in SSR mode

It means nothing - $derived works for both client and server, though it isn't reactive on the server.

Did you read the docs about testing?

[rChaoz]

though it isn't reactive on the server

Yes, that's what I meant. Isn't it reactive in my example above?

[paoloricciuti]

though it isn't reactive on the server

This could be fixed by #14977 but I think you should change your resolve conditions to browser if you also want to test $effect

[rChaoz]

This could be fixed by #14977

I don't need that, as I mentioned it is reactive for me.

I think you should change your resolve conditions to browser

I tried doing that, but I'm getting blasted with various errors, like

Error [ERR_REQUIRE_ESM]: require() of ES Module [...]\node_modules\@asamuzakjp\css-color\dist\esm\css-color.min.js from [...]\node_modules\cssstyle\lib\parsers.js not supported.
Instead change the require of css-color.min.js in [...]\node_modules\cssstyle\lib\parsers.js to a dynamic import() which is available in all CommonJS modules.

also, this makes every server test to fail.

[paoloricciuti]

Do you have other tests other than those? You need to run the svelte ones with condition browser and the others with condition node...take a look at this repo

https://github.com/dominikg/vitest-example-svelte5

[rChaoz]

I will take a look at that repo, although not sure if it will help, running just a single file with the test in my original post fails with the errors I mentioned. Also, I managed to get the transformed module, which looks good I think:

let num = __vite_ssr_import_0__.state(0);
let doubled = __vite_ssr_import_0__.derived(() => __vite_ssr_import_0__.get(num) * 2);

__vite_ssr_import_0__.set(num, 5);
__vite_ssr_import_1__.expect(__vite_ssr_import_0__.get(doubled)).toBe(10);

__vite_ssr_import_0__.effect_root(() => {
	let a = 0;

	__vite_ssr_import_0__.user_effect(() => {
		a = 5;
	});

	__vite_ssr_import_3__.flushSync();
	__vite_ssr_import_1__.expect(a).toBe(5);
});

Update: actually, I think I figured it out. Looking at the top of the generated module, I see this:

const __vite_ssr_import_0__ = await __vite_ssr_import__("/node_modules/svelte/src/internal/client/index.js");
const __vite_ssr_import_1__ = await __vite_ssr_import__("/node_modules/vitest/dist/index.js", {"importedNames":["vi","expect","it","describe"]});
const __vite_ssr_import_3__ = await __vite_ssr_import__("/node_modules/svelte/src/index-server.js", {"importedNames":["flushSync"]});

so, runes are transformed correctly, but flushSync() is imported from the server module, which is actually a no-op. I'll keep looking into this.

[rChaoz]

After upgrading to Vitest 3, I finally managed to get this working, using the example repo from above, with the workspace config.