rc-typescript

TypeScript

Not a language tutorial — the model already writes TypeScript. This is the type design the model does not apply by default: left alone it produces loose, structurally-typed code that compiles and still permits the bug. For framework and library API behaviour, which moves faster than any skill can track, use the Context7 MCP; for the conventions of a specific repo, its AGENTS.md.

The whole discipline is one idea: make the illegal state unrepresentable, so the wrong code fails to compile instead of failing in production.

Model the state, don't flag it

Booleans multiply into states that cannot occur. Three flags describe eight states when the domain has three.

// permits { loading: true, error: Error, data: User } — meaningless, and reachable
type State = { loading: boolean; error?: Error; data?: User };

// exactly the states that exist
type State =
  | { status: 'idle' }
  | { status: 'loading' }
  | { status: 'failed'; error: Error }
  | { status: 'loaded'; data: User };

The discriminated union pays off at every use site: after if (s.status === 'loaded') the compiler knows s.data exists, so the optional-chaining and the non-null assertions disappear on their own.

The tell that you need one: two optional fields where exactly one is ever set, or a comment explaining which combination is valid.

Exhaustiveness

A switch over a union must be exhaustive, and the compiler must be the thing that enforces it — so adding a variant breaks the build at every site that needs updating, instead of silently falling through.

function label(s: State): string {
  switch (s.status) {
    case 'idle':    return 'Ready';
    case 'loading': return 'Loading…';
    case 'failed':  return s.error.message;
    case 'loaded':  return s.data.name;
    default: {
      const unreachable: never = s;   // a new variant fails to compile here
      throw new Error(`unhandled: ${JSON.stringify(unreachable)}`);
    }
  }
}

A default that returns a fallback value instead of the never check is how a new variant ships unhandled.