store-class.js

export class Store {
  private subscribers: Function[];
  private reducers: { [key: string]: Function };
  private state: { [key: string]: any };

  constructor(reducers = {}, initialState = {}) {
    this.subscribers = [];
    this.reducers = reducers;
    this.state = this.reduce(initialState, {});
  }

  get value() {
    return this.state;
  }
  select(fn) {
    return fn(this.state);
  }
  subscribe(fn) {
    this.subscribers = [...this.subscribers, fn];
    fn(this.value);
    return () => {
      this.subscribers = this.subscribers.filter(sub => sub !== fn);
    };
  }

  dispatch(action) {
    this.state = this.reduce(this.state, action);
    this.subscribers.forEach(fn => fn(this.value));
  }

  private reduce(state, action) {
    const newState = {};
    for (const prop in this.reducers) {
      newState[prop] = this.reducers[prop](state[prop], action);
    }
    return newState;
  }
}


export function exampleReducer(
  state = initialState,
  action: { type: string, payload: any }
) {
  // do stuff, return state
  return state;
}

const reducers = {
  example: exampleReducer,
};

const store = new Store(reducers);


const getProducts = state => state.products;
const getPizzas = state => state.pizzas;
const getEntities = state => state.entities;

const entities$ = store.select(state =>
  getEntities(getPizzas(getProducts(state)))
);

export const initialState = {
  data: [],
  entities: {},
  loaded: false,
  loading: false,
};