useIsMounted

Obtain a stable callback that reports whether its hook is still mounted.

What it does

useIsMounted() returns an IsMounted callback. It returns true while attached and becomes false after disposal.

Capture the callback before starting delayed work, then check it before touching hook-owned state. Prefer cancellation when an API supports it; the mounted check is a final safety guard.

Signature and return type

IsMounted useIsMounted()

The hook has no parameters and returns a stable bool Function().

Usage

class DelayedSave extends HookComponent {
  const DelayedSave({super.key});

  @override
  Component build(BuildContext context) {
    final mounted = useIsMounted();
    final status = useState('idle');

    Future<void> save() async {
      await repository.save();
      if (mounted()) status.value = 'saved';
    }

    return button(onClick: save, [text(status.value)]);
  }
}

Live demo

Interactive useIsMounted demo

Mounted now: true

Ownership and lifecycle

The hook owns the mounted flag and stable callback. It reports true while attached and permanently reports false after disposal; calling the callback does not rebuild.

Server rendering

The callback is valid during SSR but long-running server work should use Jaspr's server data facilities rather than client lifecycle state.

Common mistakes

Do not use the mounted check as a substitute for cancelling timers, subscriptions, or requests. Check it immediately before a hook-owned mutation after the asynchronous boundary, not only before work starts.

Async snapshot hooks already guard their own lifecycle. Use useEffect cleanup to cancel subscriptions and timers.