redux.js

function createStore(reducer) {
  let state;
  let listeners = [];
  
  const getState = () => state;
  
  const dispatch = (action) => {
    state = reducer(state, action);
    listeners.forEach(listener => listener());
  };
  
  const subscribe = (listener) => {
    listeners.push(listener);
    
    return () => {
      listeners = listeners.filter(l => l !== listener);
    };
  };
  
  dispatch({ type: '@@redux/INIT' });
  
  return {
    getState,
    dispatch,
    subscribe
  };
};