createStore

Implement createStore(reducer, preloadedState), a minimal Redux-style store. It returns an object with three methods:

• getState() returns the current state.
• dispatch(action) computes the next state as reducer(currentState, action), stores it, then calls every subscribed listener (with no arguments), in subscription order. It returns the dispatched action.
• subscribe(listener) registers a listener and returns an unsubscribe function. Calling unsubscribe twice is harmless.

When the store is created it runs reducer(preloadedState, { type: '@@INIT' }) once to compute the initial state, so a reducer's default state argument works. The reducer must be pure; do not mutate the state in the store.

A listener that subscribes or unsubscribes while listeners are being notified must not change who is notified in that same dispatch.