Inferring Types (v10)

It is often useful to access the types of your API within your clients. For this purpose, you are able to infer the types contained in your AppRouter.

@trpc/server exports the following helper types to assist with inferring these types from the AppRouter exported by your @trpc/server router:

inferRouterInputs<TRouter>
inferRouterOutputs<TRouter>

Inferring Input & Output Types

Let's assume we have this example router:

server.ts

import { initTRPC } from '@trpc/server';
import { z } from "zod";

const t = initTRPC.create();

const appRouter = t.router({
  post: t.router({
    list: t.procedure
      .query(() => {
        // imaginary db call
        return [{ id: 1, title: 'tRPC is the best!' }];
    }),
    byId: t.procedure
      .input(z.string())
      .query((opts) => {
        // imaginary db call
        return { id: 1, title: 'tRPC is the best!' };
    }),
    create: t.procedure
      .input(z.object({ title: z.string(), text: z.string(), }))
      .mutation((opts) => {
        // imaginary db call
        return { id: 1, ...opts.input };
    }),
  }),
});

export type AppRouter = typeof appRouter;

Using the helpers, we can infer the types of our router. The following example shows how to infer the types of the post.create procedure:

client.ts

import type { inferRouterInputs, inferRouterOutputs } from '@trpc/server';
import type { AppRouter } from './server';

type RouterInput = inferRouterInputs<AppRouter>;
type RouterOutput = inferRouterOutputs<AppRouter>;

type PostCreateInput = RouterInput['post']['create'];
type PostCreateOutput = RouterOutput['post']['create'];

Infer `TRPCClientError` types

It's also useful to infer the error type for your AppRouter

client.ts

import { TRPCClientError } from '@trpc/client';
import type { AppRouter } from './server';
import { trpc } from './trpc';

export function isTRPCClientError(
  cause: unknown,
): cause is TRPCClientError<AppRouter> {
  return cause instanceof TRPCClientError;
}

async function main() {
  try {
    await trpc.post.byId.query('1');
  } catch (cause) {
    if (isTRPCClientError(cause)) {
      // `cause` is now typed as your router's `TRPCClientError`
      console.log('data', cause.data);
    } else {
      // [...]
    }
  }
}

main();