Inferring Types

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
ts
// @filename: 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;

server.ts
ts
// @filename: 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
ts
// @filename: 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 PostCreateInput = {
    title: string;
    text: string;
}
type PostCreateOutput = RouterOutput['post']['create'];
            
type PostCreateOutput = {
    title: string;
    text: string;
    id: number;
}

client.ts
ts
// @filename: 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 PostCreateInput = {
    title: string;
    text: string;
}
type PostCreateOutput = RouterOutput['post']['create'];
            
type PostCreateOutput = {
    title: string;
    text: string;
    id: number;
}

Infer TRPCClientError types

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

client.ts
ts
// @filename: 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);
                                 
(property) TRPCClientError<BuiltRouter<{ ctx: object; meta: object; errorShape: DefaultErrorShape; transformer: false; }, DecorateCreateRouterOptions<{ post: BuiltRouter<{ ctx: object; meta: object; errorShape: DefaultErrorShape; transformer: false; }, { ...; }>; }>>>.data: Maybe<DefaultErrorData>
    } else {
      // [...]
    }
  }
}
 
main();

client.ts
ts
// @filename: 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);
                                 
(property) TRPCClientError<BuiltRouter<{ ctx: object; meta: object; errorShape: DefaultErrorShape; transformer: false; }, DecorateCreateRouterOptions<{ post: BuiltRouter<{ ctx: object; meta: object; errorShape: DefaultErrorShape; transformer: false; }, { ...; }>; }>>>.data: Maybe<DefaultErrorData>
    } else {
      // [...]
    }
  }
}
 
main();