Inferring Types

It is often useful to wrap functionality of your @trpc/client or @trpc/react-query api within other functions. For this purpose, it's necessary to be able to infer input types and output types generated by your @trpc/server router.

Inference Helpers

@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>

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({
    byId: t.procedure
      .input(z.string())
      .query(({ input }) => {
        // imaginary db call
        return { id: 1, title: 'tRPC is the best!' };
    }),
    create: t.procedure
      .input(z.object({ title: z.string(), text: z.string(), }))
      .mutation(({ input }) => {
        // imaginary db call
        return { id: 1, ...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({
    byId: t.procedure
      .input(z.string())
      .query(({ input }) => {
        // imaginary db call
        return { id: 1, title: 'tRPC is the best!' };
    }),
    create: t.procedure
      .input(z.object({ title: z.string(), text: z.string(), }))
      .mutation(({ input }) => {
        // imaginary db call
        return { id: 1, ...input };
    }),
  }),
});
 
export type AppRouter = typeof appRouter;

By traversing the router object, you can infer the types of the procedures. The following example shows how to infer the types of the procedures using the example appRouter:

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 TRPClientErrors based on your router

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<CreateRouterInner<RootConfig<{ ctx: {}; meta: {}; errorShape: DefaultErrorShape; transformer: DefaultDataTransformer; }>, { post: CreateRouterInner<RootConfig<{ ctx: {}; meta: {}; errorShape: DefaultErrorShape; transformer: DefaultDataTransformer; }>, { ...; }>; }>>.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<CreateRouterInner<RootConfig<{ ctx: {}; meta: {}; errorShape: DefaultErrorShape; transformer: DefaultDataTransformer; }>, { post: CreateRouterInner<RootConfig<{ ctx: {}; meta: {}; errorShape: DefaultErrorShape; transformer: DefaultDataTransformer; }>, { ...; }>; }>>.data: Maybe<DefaultErrorData>
    } else {
      // [...]
    }
  }
}
 
main();