TanStack Start Server Function Testing

I've been trying out TanStack Start for the last few days. So far it's been decent to work with, but I ran into a huge roadblock when it comes to unit testing their server functions with vitest. Simply calling the function in a test was throwing an error.

Error: No Start context found in AsyncLocalStorage. Make sure you are using the function within the server runtime.

I couldn't find anything in the official docs, but eventually I saw this workaround after digging around on GitHub to see how other people were tackling this. The main issue is that you TanStack Start expects a lot of metadata (Start context) to come along with a server function invocation, and so we have to supply all of that context ourselves.

Assume you have a TanStack createServerFn:

import { createServerFn } from "@tanstack/react-start";

export const getServerNumber = createServerFn({
  method: "GET",
}).handler(async () => {
  return 15;
});

I then added this function into my test library code to add the metadata.

import { runWithStartContext } from "@tanstack/start-storage-context";

export function runTestServerFn<T>(fn: () => Promise<T>) {
  // Server functions require a Start context to be available in AsyncLocalStorage
  // This mimics what TanStack Start does during request handling
  return runWithStartContext(
    {
      getRouter: () => ({}) as any,
      request: new Request("http://localhost/"),
      startOptions: {},
      contextAfterGlobalMiddlewares: {},
      executedRequestMiddlewares: new Set(),
    },
    fn,
  );
}

Then in a *.test.ts file:

it("runs ssr function", async () => {
  const result = await runTestServerFn(() => getServerNumber());
  expect(result).toEqual(15);
});

I hope TanStack looks at adding better support for testing when they go into 1.0 (to be fair, Start is in RC at the moment). It seems like a huge miss in the library to not at least have docs about best practices for testing SSRs.