Server-Side Unit Tests

tRPC offers a handy createCaller() to directly call your procedures without spinning up an Express server. Combine this with a test DB or mocking:

// server/test/userRouter.test.ts
import { test, expect, beforeEach } from 'vitest';
import { appRouter } from '../src/index';
import Database from 'better-sqlite3';

let db: Database;

beforeEach(() => {
	db = new Database(':memory:');
	db.prepare('CREATE TABLE user (id INTEGER PRIMARY KEY, name TEXT, password TEXT)').run();
	db.prepare('INSERT INTO user (name, password) VALUES (?, ?)').run('Alice', 'pass123');
});

test('getUser finds existing user', async () => {
	const caller = appRouter.createCaller({ db, user: null });
	const user = await caller.user.getUser(1);
	expect(user?.name).toBe('Alice');
});

No messing with HTTP calls, so it’s fast. If you want integration or end-to-end tests, you can spin up your server and query it normally. For big apps, you might also do front-end integration tests using Cypress or Playwright.

Debugging

• Check logs: Add an onError to the tRPC middleware or use a logging middleware.
• React Query DevTools: If using React, the dev tools can reveal query states, caching, and more.
• Console logs: When in doubt, sprinkle them around (and remove them once you’re done!).
• CORS: Make sure you enable it if your client is served from a different origin.