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Why Custom Error Types?

Basic Custom Error Class

class MyCustomError extends Error {
  constructor(message: string) {
    super(message);
    this.name = 'MyCustomError';
  }
}

function main() {
  try {
    throw new MyCustomError('Something specific went wrong!');
  } catch (error) {
    if (error instanceof MyCustomError) {
      console.log(`Caught: ${error.name} - ${error.message}`);
    } else {
      console.log(`Caught generic error: ${error.message}`);
    }
  }
}

main();

tRPC's TRPCError

Defining a tRPC Custom Error

import { TRPCError } from '@trpc/server';

class UserNotFoundError extends TRPCError {
  constructor(userId: string) {
    super({
      code: 'NOT_FOUND',
      message: `User with ID '${userId}' not found.`
    });
    this.name = 'UserNotFoundError';
  }
}

function main() {
  try {
    throw new UserNotFoundError('user-123');
  } catch (error) {
    if (error instanceof TRPCError) {
      console.log(`Error Code: ${error.code}`);
      console.log(`Error Message: ${error.message}`);
    }
  }
}

main();

Throwing Custom Errors (Backend)

import { publicProcedure, router } from './trpc'; // Assume trpc setup
import { TRPCError } from '@trpc/server';

class UserNotFoundError extends TRPCError {
  constructor(userId: string) {
    super({ code: 'NOT_FOUND', message: `User ${userId} not found.` });
    this.name = 'UserNotFoundError';
  }
}

const appRouter = router({
  getUser: publicProcedure
    .input(z.string())
    .query(async ({ input: userId }) => {
      // Simulate database lookup
      if (userId === 'nonexistent') {
        throw new UserNotFoundError(userId); // Throw our custom error!
      }
      return { id: userId, name: `User ${userId}` };
    }),
});

// Note: `z` for Zod input validation is assumed here
// The router itself is not runnable without a full server context.

Frontend: Receiving Errors

Frontend: Identifying Custom Errors

Frontend Example: Handling UserNotFoundError

import { trpc } from './utils/trpc'; // Assume trpc client setup

function UserProfile({ userId }: { userId: string }) {
  const { data, error, isLoading } = trpc.getUser.useQuery(userId);

  if (isLoading) {
    return '<p>Loading user data...</p>';
  }

  if (error) {
    if (error.data?.code === 'NOT_FOUND') {
      return `<p>User with ID <b>${userId}</b> does not exist.</p>`;
    } else {
      return `<p>An unexpected error occurred: ${error.message}</p>`;
    }
  }

  return `<h1>Welcome, ${data?.name}!</h1>`;
}

function main() {
  // This function simulates component usage.
  // In a real app, trpc.getUser.useQuery would trigger an API call.
  console.log('Simulating UserProfile for existing user...');
  // Assume UserProfile('user-123') would render 'Welcome, User user-123!'

  console.log('Simulating UserProfile for nonexistent user...');
  // Assume UserProfile('nonexistent') would render 'User with ID nonexistent does not exist.'
}

main();

Quick Check

Recap: Custom Error Types