The essence of redux in 30 lines of code.

redux.ts

class Store<S> {
  private readonly reducer: (state: S, action: Action) => S;
  private state: S;
  private listeners: Listener[] = [];

  constructor(reducer, state) {
    this.reducer = reducer;
    this.state = state;
  }

  getState(): S {
    return this.state;
  }

  dispatch(action: Action): void {
    this.state = this.reducer(this.state, action);
    this.listeners.forEach(it => it());
  }

  subscribe(listener: Listener): Unsubscribe {
    this.listeners = [...this.listeners, listener];
    return () => {
      this.listeners = this.listeners.filter(it => it !== listener);
    };
  }
}

type Action = { type: string; payload: any };
type Listener = () => void;
type Unsubscribe = () => void;

Your listeners should be provided this.state on line 17.

@jordrake That is not how redux is implemented:
https://github.com/reduxjs/redux/blob/master/src/createStore.js#L212

See: https://redux.js.org/faq/design-decisions#why-doesn-t-redux-pass-the-state-and-action-to-subscribers

Rather than having the reducer passed into the constructor, would it make sense to have a reducers array and an addReducer(s) and removeReducer(s) function?
It just seems unlikely that a complex system would only ever have one, fixed reducer function for the entire State. More likely that modules would want to add reducers to modify state when they are instantiated, then remove their reducers when they were done.