subscribe

subscribe() registers a listener and returns a cleanup function.

subscribe(listener: () => void): () => void

The listener is called after state changes. It receives no arguments, so call getState() inside the listener when you need the latest value.

Basic usage

const unsubscribe = store.subscribe(() => {
  console.log(store.getState());
});

store.setState((prev) => ({ ...prev, ready: true }));
unsubscribe();

Cleanup is part of the contract

Every subscribe() call adds the listener to an internal Set. If you never call the returned unsubscribe function, that listener remains registered.

Connect cleanup to the owner lifecycle:

function connectPanel(panel: { render(value: unknown): void; destroy(cb: () => void): void }) {
  const unsubscribe = store.subscribe(() => {
    panel.render(store.getState());
  });

  panel.destroy(unsubscribe);
}

Notification timing

Listeners run synchronously after the state is stored. A listener added during notification is not called for the current notification cycle because the store iterates over Array.from(this.listeners). A listener removed during notification may still run if it was already copied into that cycle's array.

No notification for same value

If setState() resolves to the same value or same reference by Object.is, listeners do not run.

const unsubscribe = store.subscribe(() => {
  console.log("changed");
});

store.setState((prev) => prev); // no log
unsubscribe();

Listener errors

The store does not catch listener errors. If one listener throws, the setState() call throws and later listeners in that notification cycle may not run. Keep listeners small or catch expected errors inside your listener.