As long as an object correctly implements the subscribe method, it's a store. Beyond that, anything goes. It's very easy, therefore, to create custom stores with domain-specific logic.

For example, the count store from our earlier example could include increment, decrement and reset methods and avoid exposing set and update:

function createCount() {
  const { subscribe, set, update } = writable(0);

  return {
    subscribe,
    increment: () => update(n => n + 1),
    decrement: () => update(n => n - 1),
    reset: () => set(0)
  };
}