trpc-koa-adapter

This is an adapter which allows you to mount tRPC onto a Koa server. This is similar to the trpc/packages/server/src/adapters/express.ts adapter.

How to Add tRPC to a Koa Server

Initialize a tRPC router and pass into createKoaMiddleware (along with other desired options). Here is a minimal example:

import Koa from 'koa';
import { createKoaMiddleware } from 'trpc-koa-adapter';
import { initTRPC } from '@trpc/server';

const ALL_USERS = [
  { id: 1, name: 'bob' },
  { id: 2, name: 'alice' },
];

const trpc = initTRPC.create();
const trpcRouter = trpc.router({
  user: trpc.procedure
    .input(Number)
    .output(Object)
    .query((req) => {
      return ALL_USERS.find((user) => req.input === user.id);
    }),
});

const app = new Koa();
const adapter = createKoaMiddleware({
  router: trpcRouter,
  prefix: '/trpc',
});
app.use(adapter);
app.listen(4000);

You can now reach the endpoint with:

curl -X GET "http://localhost:4000/trpc/user?input=1" -H 'content-type: application/json'

Returns:

{ "id": 1, "name": "bob" }

createKoaMiddleware Arguments

The middleware takes a configuration object with the following properties:

Option	Required	Description
router	Required	The tRPC router to mount
createContext	Optional	A function returning the tRPC context. If defined, the type should be registered on tRPC initialization as shown in an example below and the tRPC docs: https://trpc.io/docs/context
prefix	Optional	The prefix for tRPC routes, such as /trpc
nodeHTTPRequestHandler options	Optional	Any of the options used by the underlying request handler. See tRPC's nodeHTTPRequestHandler for more details

More examples

In addition to these examples, see /example and the implementations in /test/createKoaMiddleware.test.ts.

Using the Context:

const createContext = async ({ req, res }: CreateTrpcKoaContextOptions) => {
  return {
    req,
    res,
    isAuthed: () => req.headers.authorization === 'trustme',
  };
};

type TrpcContext = inferAsyncReturnType<typeof createContext>;

const trpc = initTRPC.context<TrpcContext>().create();

const trpcRouter = trpc.router({
  createUser: trpc.procedure.input(Object).mutation(({ input, ctx }) => {
    // ctx should be fully typed here
    if (!ctx.isAuthed()) {
      ctx.res.statusCode = 401;
      return;
    }

    const newUser = { id: Math.random(), name: input.name };
    ALL_USERS.push(newUser);

    return newUser;
  }),
});

const adapter = createKoaMiddleware({
  router: trpcRouter,
  createContext,
  prefix: '/trpc',
});

Note About Using With a Body Parser:

Using a bodyparser such as @koa/bodyparser, koa-bodyparser, or otherwise parsing the body will consume the data stream on the incoming request. To ensure that tRPC can handle the request, this library looks for the parsed body on ctx.request.body, which is where @koa/bodyparser and koa-bodyparser store the parsed body. If for some reason the parsed body is being stored somewhere else, and you need to parse the body before this middleware, the body will not be available to tRPC and mutations will fail as detailed in this github issue.

Development

The project uses pnpm for package management.

To get started clone the repo, install packages, build, and ensure tests pass:

git clone https://github.com/BlairCurrey/trpc-koa-adapter.git
cd trpc-koa-adapter
pnpm i
pnpm build
pnpm test

Git commit messages must follow conventional commit standard (enforced by husky hooks). Versioning is handle by github actions and is determined by commit messages according to the semantic-release rules and .releaserc configuration.