How to correctly type and access optional middleware variables in Hono context?

[DanielBoxer]

When you have middleware that sets a variable in the context that's only used in some routes, how are you supposed to tell TypeScript that the variable is available in the routes using that middleware?

The best approach I've found (in discussion #3257) is to mark the variable as optional in the context type:

export type AppContext = {
  Bindings: CloudflareBindings;
  Variables: {
    db?: Kysely<DB>;
  };
};

Then I have a helper function to access it which imports the AppContext type:

export const useDb = createMiddleware<AppContext>(async (c, next) => {
  const db = getDb();
  c.set('db', db);

  try {
    await next();
  } finally {
    c.executionCtx.waitUntil(db.destroy());
  }
});

export function requireDb(c: Context<AppContext>): Kysely<DB> {
  const db = c.var.db;
  if (!db) {
    throw new HTTPException(500, { message: 'Database is missing from context' });
  }
  return db;
}

This works, but I was wondering if there's a better way to do this in Hono?