How to test Server Functions

[cameronb23]

I believe that server functions, just like any other functions or unit of work, should be able to be well tested. Currently, this doesn't work out of the box as expected (via Vitest) due to some inherent checks that are made and errors thrown when creating the server function:

Error: Invariant failed: createServerFn must be called with a function that is marked with the 'use server' pragma. Are you using the @tanstack/router-plugin/vite ?

While this is likely necessary for Start, there should be some official method or guidance on how to test these functions. I could imagine there being a change made to createServerFn to ignore these checks if the environment is test or something, or this may be as simple as adding a section to the documentation. For now, the best that I have come up with is mocking the call globally - this is an example for Vitest, but I assume it would be similar for Jest and others.

// vitest-setup.ts
import type { Fetcher, FetchFn } from '@tanstack/start';
import { vi } from 'vitest';

export default function () {
  vi.mock(import('@tanstack/start'), async (importOriginal) => {
    const original = await importOriginal();

    return {
      ...original,
      createServerFn: <TMethod extends 'GET' | 'POST', TPayload, TResponse>(
        _ignoredMethod: TMethod,
        fn: FetchFn<TPayload, TResponse>,
      ) => fn as Fetcher<TPayload, TResponse>,
    };
  });
}

// vitest.config.ts
import { defineConfig } from 'vitest/config';
import tsconfigPaths from 'vite-tsconfig-paths';

export default defineConfig({
  plugins: [tsconfigPaths({ projects: ['./tsconfig.json'] })],
  test: {
    setupFiles: 'vitest-setup.ts',
    // ... Specify options here.
  },
});