Building a Custom Store with useSyncExternalStore

A custom store needs three things: some state, a subscribe method to register listeners, and a getSnapshot method to read the current state. That is the full contract useSyncExternalStore expects.

Keeping the store this small makes it framework-agnostic, so the same store can be used inside or outside React.

A createStore factory takes an initial value and returns an object with getSnapshot, subscribe, and an update method. Internally it holds the current state and a set of listener callbacks.

This factory pattern lets you spin up multiple independent stores with a single reusable function.