honojs-middleware/packages/trpc-server/src/index.ts

import type { AnyRouter } from '@trpc/server'
import type { FetchCreateContextFnOptions, FetchHandlerRequestOptions } from '@trpc/server/adapters/fetch'
import { fetchRequestHandler } from '@trpc/server/adapters/fetch'
import type { Context, MiddlewareHandler } from 'hono'

type tRPCOptions = Omit<FetchHandlerRequestOptions<AnyRouter>, 'req' | 'endpoint' | 'createContext'> &
  Partial<Pick<FetchHandlerRequestOptions<AnyRouter>, 'endpoint'>> &
  { createContext? (opts: FetchCreateContextFnOptions, c: Context): Record<string, unknown> }

export const trpcServer = ({ endpoint = '/trpc', createContext, ...rest }: tRPCOptions): MiddlewareHandler => {
  return async (c) => {
    const res = fetchRequestHandler({
      ...rest,
      createContext: (opts) => ({
        ...createContext ? createContext(opts, c) : {},
        // propagate env by default
        env: c.env,
      }),
      endpoint,
      req: c.req.raw,
    })
    return res
  }
}