Cofounder Testing

Automated Testing

Tests are written in tandem with the code they verify. Create handler → create test file. Create interactive component → create test file.

The Two Layers

Layer 1 tests the API contract against a real database. Layer 2 tests UI logic, rendering, and user interactions with mocked API calls.

Running commands

All invocations must use mise x -- (e.g., mise x -- go test, mise x -- npx vitest). Never invoke tools directly.

Setup Sequence

When starting a new project, set up both layers as the first features are being built. For each layer, look up the current recommended test runner and libraries for the project's language and framework, then configure accordingly. The sections below describe what to test and how much — not which specific library to use.

Layer 1: Backend unit and integration tests

Go has a built-in test runner.

Coverage expectations

• Every handler gets at least one test covering the happy path.
• Database-touching code gets integration tests that run against the real local database — not mocks. Use a test helper that runs migrations and wraps each test in a transaction that rolls back.
• HTTP handlers are tested by sending real HTTP requests to a test server and asserting on status codes and response bodies.
• Use table-driven tests. Each test case gets a descriptive name.

Colocated test files

Test files live next to the code they test. For example, handler/todo_test.go tests handler/todo.go. When creating a new handler file, immediately create the corresponding test file.

Retrofitting tests to an existing codebase

1. List all handler files that don't have a corresponding test file.
2. Create a test file for each, starting with handlers that touch the database or have complex logic.
3. Simple pass-through handlers (e.g., health check returning 200) can be skipped.

Layer 2: Frontend component tests

Component tests render individual React components in a simulated DOM, simulate user interactions, and assert on visible output. They catch UI logic bugs — conditional rendering, form validation, state management — without needing a running backend.

Coverage expectations

• Components with non-trivial logic (conditional rendering, form handling, computed state, event handlers) get tests.
• Pure presentational components that just render props without logic do not need tests.
• Pages that are layout wrappers or only compose other already-tested components do not need their own tests.

Principles

• Test behavior, not implementation. Render the component → simulate what a user would do (click, type, submit) → assert on what the user would see. Don't assert on internal state or implementation details.
• Mock external dependencies. API calls, router context, and other services should be mocked so component tests run without a backend.
• Colocated test files. Test files live next to the component they test (e.g., TodoList.test.tsx next to TodoList.tsx).

Example — the kind of test to write for a component with interactive logic:

// Render with props → simulate a click → assert visible result changed
test('marks a todo as complete when clicked', async () => {
  render(<TodoList items={[{ id: 1, text: 'Buy milk', done: false }]} />)
  await user.click(screen.getByText('Buy milk'))
  expect(screen.getByText('Buy milk')).toHaveClass('completed')
})

The specific API depends on the test runner and testing library in use. The pattern — render, act, assert on visible output — is universal.

Setup

Choose a test runner compatible with the frontend framework (e.g., Vitest for Vite-based projects) and a DOM testing library that encourages testing from the user's perspective. Configure a simulated DOM environment (jsdom or similar) and add a test script to package.json. Look up the current recommended setup for the project's toolchain.

Vitest + Vite config: Import defineConfig from 'vitest/config', not 'vite'. This is the only way to make the type check accept the test block. Do not use /// <reference types="vitest" /> — it doesn't fix the overload signature and tsc will fail with TS2769.

No @vitejs/plugin-react for Vitest. In React Router framework mode it conflicts with the router's toolchain (incompatible Babel), and Vitest transforms JSX natively — add no JSX transform config at all.

Retrofitting tests to existing components

1. Identify components with non-trivial logic.
2. Create test files for those components first.
3. Add data-testid attributes to key interactive elements for stable selectors.

TypeScript type check — required before every commit

The dev server skips type checking for speed, and the production build (react-router build) doesn't type-check either — so errors like unused imports, type mismatches, or config file issues never surface on their own. Always run the type check before committing:

bash -c 'cd "$(git rev-parse --show-toplevel)/frontend" && mise x -- npm run typecheck'

The scaffold's typecheck script runs react-router typegen && tsc — the typegen step generates the route types in frontend/.react-router/ that tsc needs. Never run bare tsc directly; on a fresh checkout it fails without the generated types.

Coverage Catch-Up

After running tests, scan for gaps: backend handlers without _test.go, frontend components with logic but no .test.tsx. Report gaps to the user and offer to add missing tests.