Question: Is createIsomorphicFn intended to work at module top-level?

[mountain1009]

Question

Is createIsomorphicFn intended to work when called at module top-level?

I noticed that it returns a function instead of the expected value when called at module top-level, but works correctly when called inside a function scope (components, server functions, loaders).

Reproduction

https://github.com/mountain1009/tanstack-start-isomorphic-bug

pnpm install
pnpm dev
# Open http://localhost:5173

Observed Behavior

Call Location	Result	Status
Module top-level	Returns [Function]	❓
Inside Component	Returns "server" / "client"	✅ Works
Inside Server Function	Returns "server"	✅ Works
Inside Loader	Returns "server"	✅ Works

Code Example

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

const getEnvironment = createIsomorphicFn()
  .server(() => "server")
  .client(() => "client");

// Module top-level: returns [Function] instead of "server"
const moduleLevel = getEnvironment();
console.log(typeof moduleLevel); // "function" ❓

function MyComponent() {
  // Inside function: works correctly
  const env = getEnvironment();
  console.log(typeof env); // "string" ✅
}

My Use Case

I was trying to initialize an oRPC client at module top-level:

const getORPCClient = createIsomorphicFn()
  .server(() => createRouterClient(...))
  .client(() => createORPCClient(...));

const client = getORPCClient(); // Returns function, not client
export const orpc = createTanstackQueryUtils(client); // Fails

Questions

1. Is module top-level usage supported?
2. If not, could this limitation be documented?
3. Is there a recommended pattern for initializing isomorphic singletons?

Workaround

Using manual environment detection works:

function getORPCClient() {
  if (typeof window === "undefined") {
    return createRouterClient(...);
  }
  return createORPCClient(...);
}

Environment

• @tanstack/react-start: 1.143.6
• @cloudflare/vite-plugin: 1.0.0+
• Vite: 7.0.0

Thank you!

[schiller-manuel]

can you please provide a complete reproducer as a git repo?

[mountain1009]

@schiller-manuel
Thanks for the quick response!

I've added a complete reproducer: https://github.com/mountain1009/tanstack-start-isomorphic-bug

After further testing, I found that createIsomorphicFn works correctly inside function scopes (components, server functions, loaders) but returns a function when called at module top-level.

I've updated the issue to a question format since I'm now unsure if module top-level usage is intended to be supported.