Prevent undefined/void/falsy values from becoming accidental runtime states.

Apply these rules:

1. Eliminate transient undefined: If a value can be absent (e.g., localStorage, optional query data), wire an explicit defaultValue so hooks/state initialize to a concrete value and consumers never render an “undefined first frame”.
   • Prefer setting the useState initial value from defaultValue inside the utility hook rather than letting it be undefined and patching later.
2. Guard globals by environment: Never assume browser APIs exist—use a shared “is server/client” check before touching window/DOM.
3. Treat void/no-arg and falsy as testable cases: When designing/using optional variables or cached data, add type and runtime tests for void (no variables) and for falsy cached values (e.g., '', 0, false) so “absent” isn’t conflated with “no result”.
4. Don’t rely on undefined being “missing” during key/hash matching: If keys are serialized/hashed (e.g., via JSON.stringify) or displayed via hashes, undefined may be removed, making different logical keys collide or fail to match.
   • Prefer key shapes where absence is explicit (e.g., use a dedicated sentinel or omit the property at the call site) and document/align hashing with comparison semantics.

Example (default to avoid initial undefined):

function useLocalStorage<T>(key: string, defaultValue: T) {
  const [value, setValue] = React.useState<T>(() => {
    const raw = localStorage.getItem(key)
    return raw == null ? defaultValue : (JSON.parse(raw) as T)
  })
  // ...sync back to localStorage
  return [value, setValue] as const
}

If a behavior constraint exists (e.g., a function must not return undefined), encode it in types and tests—don’t “test” impossible states.