Redux in a nutshell

mini-redux.js

function createStore (reducers) {
  let state = reducers()
  const subscribers = [];
  const store = {
    dispatch: (action) => {
      state = reducers(state, action);
      subscribers.forEach((subscriber) => subscriber());
    },
    getState: () => {
      return state
    },
    subscribe: (subscriber) => {
      subscribers.push(subscriber);
    }
  }

  return store;
}

const reducers = (state = { counter: 0 }, action) => {
  if (!action) { return state; }
  switch (action.type) {
    case 'INCREMENT':
      return { counter: state.counter + 1  };
    case 'DECREMENT':
      return { counter: state.counter - 1  };
    default:
      return state;
  }
}

const store = createStore(reducers);

store.subscribe(() =>
  console.log(store.getState());
);

store.getState(); // => { counter: 0 }
store.dispatch({ type: 'INCREMENT' })
store.getState(); // => { counter: 1 }