This gist is a very stripped out implementation of the redux library.

boiledDownReduxImplementation.js

function createStore(reducer) {
    let state;
    let subscribers = [];

    function getState() {
        return state;
    }

    function dispatch(action) {
        state = reducer(state, action);

        //update the state and then call each of the subscriber.
        subscribers.forEach((subscriber) => subscriber());
    }

    function subscribe(subscriber) {
        subscribers.push(subscriber);

        // function to unsubscribe a particular subscribed function.
        return function () {
            subscribers = subscribers.filter(function (itemSubscriber) {
                return subscriber !== itemSubscriber;
            });
        };
    }

    function replaceReducer(newReducer) {
        // replace the original reducer
        reducer = newReducer;
    }
    
    // this call to dispatch is made to initialize the state object because reducer would contain a default initial state.
    dispatch({});
    return { getState, dispatch, subscribe, replaceReducer };
}