[Start] Function middleware: try/catch doesn't catch errors from next() - must check result.error instead

[alexander-zuev]

Description

Function middleware (type: 'function') cannot catch errors from inner middleware/handlers using try/catch. The docs show this pattern, but it doesn't work for function middleware.

Reproduction

// This does NOT work - catch block never fires
const errorMiddleware = createMiddleware({ type: 'function' }).server(async ({ next }) => {
  try {
    return await next()
  } catch (error) {
    // Never reached when inner middleware/handler throws
    console.log('caught', error)
    throw error
  }
})

Root Cause

In createServerFn.js, userNext catches errors and returns them as { error } instead of throwing:

// Line 154-161
const userNext = async (userCtx = {}) => {
  try {
    return await callNextMiddleware(nextCtx);
  } catch (error) {
    return { ...nextCtx, error };  // Returns, doesn't throw
  }
};

Working Solution

Must check result.error instead:

const errorMiddleware = createMiddleware({ type: 'function' }).server(async ({ next }) => {
  const result = await next()
  
  if (result.error) {
    // Handle error here
    return { ...result, error: new Error('Sanitized message') }
  }
  
  return result
})

Issues

1. Docs show try/catch (observability guide) but that's for request middleware, not function middleware
2. TypeScript types don't include error on the result - requires cast: (await next()) as { error?: Error }
3. Ambiguity - unclear when to use try/catch vs result.error

Questions

1. Is checking result.error the intended pattern for function middleware error handling?
2. Should TypeScript types include error property on FunctionServerResultWithContext?
3. Should docs clarify the difference between request middleware and function middleware error handling?

Environment

• @tanstack/react-start: 1.149.1
• @tanstack/start-client-core: 1.149.1

[alexander-zuev]

This was fixed in v1.151.3 via #6392. Closing as resolved. Thanks!