BillTracker/docs/CODE_STANDARDS.md

# Code Standards

The living rulebook for this codebase. One canonical way to do each thing, enforced in
CI so it can't drift. Sections marked **(enforced)** fail CI; **(target)** are being
rolled out. Update this doc when a standard changes.

## Tooling & enforcement (enforced)

- **Formatting:** Prettier (`.prettierrc.json`) — single quotes, semicolons, 2-space,
  `printWidth: 100`, trailing commas. Run `npm run format`; CI runs `format:check`.
- **Lint:** `npm run lint` (`eslint .`) covers **client and server**. Focused rule set
  (react-hooks on the client; `js/recommended` + TS-unused on the server). Errors block CI;
  warnings are advisory (ratchet down over time).
- **Typecheck:** `npm run typecheck` (client, strict + `noUncheckedIndexedAccess`) and
  `npm run typecheck:server` (server). Both run in CI.
- **Local hooks (lefthook):** pre-commit auto-formats staged files + lints; pre-push
  typechecks. Installed by `npm install` (the `prepare` script).
- **Dead code:** `npm run knip` (advisory) for unused exports/deps.
- **Full gate:** `npm run ci` = format:check + lint + typecheck (×2) + check:server +
  tests (server + client) + build. The CI pipeline runs the same.

## Modules & files

- Server is **CommonJS TypeScript `.cts`**, run by Node via type-stripping (no build step).
  Every migrated module is `require`d **with the explicit `.cts`/`.mts` extension**
  (extensionless does not resolve). Each `.cts` leads with an `import type …` line so
  type-stripping activates.
- Client is ESM `.ts`/`.tsx` with the `@/*` path alias.
- Money is **integer cents** in the DB and on the wire; convert to dollars only at the edge
  via the money helpers. Branded `Cents`/`Dollars` types where practical.

## HTTP API conventions

- **Wire format:** JSON, **snake_case** field names (mirrors DB columns), money as integer
  cents.
- **Error responses (enforced):** every error body is `{ error, message, code, field? }` with
  the correct HTTP status. Throw the factories from `utils/apiError.cts`
  (`ValidationError`, `NotFoundError`, `ConflictError`, `AuthError`, `ForbiddenError`, or
  `new ApiError(code, message, status, { field })` for anything else) and let the single
  terminal error handler format them — do not hand-roll `res.status().json({ error })`.
  Express 5 forwards rejected async handlers automatically, so route bodies do not wrap
  everything in `try/catch`; a thrown `ApiError` keeps its message on the wire while any
  other 5xx is masked + logged by the terminal handler (that masking is the whitelist:
  wrap a message in `ApiError` only when it is deliberately user-facing). Clients read
  `message`/`code` + status. **Canonical example:** `routes/categories.cts`.
  **Enforced by:** the eslint `no-restricted-imports` ratchet on `routes/**` (all 29 route
  files converted 2026-07). **Documented exceptions** (each commented at the site):
  `routes/transactions.cts` (internal `{ error }` parse-result convention, rethrown at the
  boundary via `throwStandardized`), `routes/import.cts` (`sendImportError` error-id
  envelope), `routes/admin.cts` `sendError` (backup download can fail mid-stream after
  headers are sent), payments' `DUPLICATE_SUSPECTED` 409 and transactions' bulk-unmatch 500
  (both carry extra payload fields the client renders), `routes/calendarFeed.cts`
  (text/plain for ICS clients).
- **Client error shape:** `client/api.ts` builds every thrown error through `toApiError()` —
  one construction site; on a 401 outside `/auth/*` it dispatches `auth:expired` so
  `AuthProvider`/`RequireAuth` redirect to login. Mutations follow the React Query patterns
  in `client/hooks/useQueries.ts` / `usePaymentActions.ts` (invalidate + toast on error).
- **Success envelope (target):** reads return the bare resource/list; mutations return
  `{ success: true, … }`.
- **Validation (target):** validate inputs with the shared validators (throwing
  `ValidationError`); do not scatter ad-hoc `parseInt`/range checks.

## Security (target; see also SECURITY.md)

- Every **mutating** route carries CSRF + auth + an appropriate rate-limit + input
  validation. Admin routes require `requireAdmin`.
- Every user-scoped query filters `user_id` **and** `deleted_at IS NULL`. Never interpolate
  user input into SQL — use bound parameters.
- Never log secrets, tokens, or raw financial values.
- Fail fast at boot on missing required config (e.g. `TOKEN_ENCRYPTION_KEY` in production).

## Logging (enforced)

- Use `utils/logger.cts` (`const { log } = require('.../utils/logger.cts')`) — `log.debug/info/warn/error`,
  console-compatible. Level via `LOG_LEVEL` (default: debug in dev, info in prod). It **redacts**
  secrets/PII (sensitive keys, bearer/JWT) so tokens/passwords never reach logs. All server
  runtime `console.*` has been migrated. Do **not** use raw `console.*` in server code.

## Data integrity

- Every **multi-statement write** runs inside `db.transaction()`.
- Migrations are append-only versioned entries in `db/migrations/versionedMigrations.cts`
  (idempotent; guard column adds with `PRAGMA table_info`).

## Testing

- Server: `node:test` in `tests/*.test.js`. Client: Vitest (`*.test.ts[x]`). E2E: Playwright
  (`npm run test:e2e`, probe subset `test:e2e:probe`). Add coverage for new handlers; prove
  money invariants with property tests where practical.

## Typed SQL (Kysely — available, adopt incrementally)

`db/kysely.cts` provides Kysely as a **type-safe query builder executed synchronously**.
The app is sync (better-sqlite3) and Kysely's `.execute()` is async, so we never execute
through Kysely — we build the query (columns + `user_id`/`deleted_at` scoping + types all
checked at compile time), then run it via the sync helpers:

```ts
const { kysely, all, get, run } = require('../db/kysely.cts');
const cats = all(
  kysely()
    .selectFrom('categories')
    .select(['id', 'name'])
    .where('user_id', '=', userId)
    .where('deleted_at', 'is', null),
);
```

Compiled SQL is always parameterized (injection-safe). Prefer this for **new/refactored**
user-scoped queries; extend the `Database` schema in `db/kysely.cts` as tables are adopted.
Proven byte-identical to raw SQL in `tests/kyselyPoc.test.js`.

## Future / modernization (tracked)

Zod (validation) · `node:sqlite` (drop the native dep) · ts-rest + OpenAPI · Vite 6/7.
Adopt where each makes the code more uniform; see the standardization plan.