tsc kept saying 0 errors — because it had stopped checking anything

Background

bopomofokids.com is a static React SPA I built for my kid — no backend, no test framework beyond Playwright e2e and Vite’s build. Its lint script is just tsc --noEmit; there’s no ESLint, no separate type-coverage tool. That one command is the entire static-analysis safety net for the project.

The tsconfig.json has neither strict nor noImplicitAny turned on — a deliberate choice to keep a solo side project’s type checking lightweight rather than importing a large ruleset for a codebase with one contributor. That looseness is exactly what let this go unnoticed.

What actually happened

At some point @types/react and @types/react-dom went missing from node_modules — not through any change to package.json, just an environment where they hadn’t actually been installed. npm run lint kept running. It kept exiting 0.

That’s the surprising part: a missing type package for a module you import everywhere should be the loudest possible failure. It wasn’t, because of how TypeScript resolves an import it can’t find a declaration for.

Concretely, this quietly disabled:

// useCallback's generic inference — the callback's param/return types
// stop being inferred or checked; TypeScript just accepts whatever's passed
const onSelect = useCallback((symbol: BopomofoItem) => { ... }, []);

// React.FC<Props> — prop type checking on every component
// with 'React' resolved to any, the whole generic is inert
const QuizCard: React.FC<QuizCardProps> = ({ item, onPick }) => { ... };

// JSX itself — a typo'd prop, a nonexistent one, a wrong type,
// none of it gets flagged once the JSX namespace is untyped
      what `any` silently swallows
    

A typo’d prop name, a component given the wrong shape of data, a hook used with the wrong argument type — all of these are exactly the class of bug tsc --noEmit exists to catch, and all of them stopped being caught, with no error, warning, or exit-code change to say so.

Why skipLibCheck doesn’t explain this away

This project’s tsconfig.json has skipLibCheck: true, which sounds related but isn’t the cause. skipLibCheck only skips checking the contents of .d.ts files that do exist — it doesn’t fabricate type declarations for a package that’s missing entirely. The failure here is one level earlier: there was no .d.ts to skip checking, so TypeScript fell back to its default answer for an import with no available types, which is any.

How something like this gets noticed at all

Nothing about a CI run flags this. “0 errors” prints identically whether the codebase is genuinely clean or the type checker has quietly stopped looking at anything React-shaped. The only tell is behavioral: tsc feeling strangely untroubled by a mistake that should obviously fail — a prop that doesn’t exist on a component, a hook called with the wrong type — is the signal to check the checker’s own inputs, not the code it’s supposedly checking.

Takeaways