Simple Redux implementation - final

createStore.js

const createStore = (reducer, initialState = {}, middlewares = []) => {
    let state = initialState;
    const listeners = [];

    // final step - call the reducer and invoke the listeners
    let finalMiddleware = (action) => {
      state = reducer(state, action);
      listeners.forEach(listener => listener());
      return state;
    }
    // the middleware API is constructed of getState and dispatch
    const middlewareAPI = { getState: () => state, dispatch: (action) => finalMiddleware(action) }
    // evalutate the first function in the structure of the middlewares
    const newMiddleWares = middlewares.map(middleware => middleware(middlewareAPI));
    // go over the new middlewares reversed and evaluate the second function in the structure
    // the second function receives the third fucntion of the next middleware
    // this way we can make sure that when the developer will call next(action)
    // he will acually execute the next middleware's most inner funciton
    for(let i = newMiddleWares.length - 1; i >= 0; i--) {
      finalMiddleware = newMiddleWares[i](finalMiddleware);
    }

    return {
      ...middlewareAPI,
      subscribe: (listener) => {
        listeners.push(listener);
        return () => {listeners.splice(listeners.indexOf(listener), 1);}}
    };
}