reduxtest.js

const { createStore } = require('redux');

const initialState = {
  age: 21
};

const myReducer = (state = initialState, action) => {
  let newState = { ...state };
  if (action.type === 'ADD') {
    newState.age += 1;
  }

  if (action.type === 'SUBSTRACT') {
    newState.age -= 1;
  }

  return newState;
};

const store = createStore(myReducer);

store.subscribe(() => {
  console.log('store change: ' + JSON.stringify(store.getState()));
});

store.dispatch({ type: 'ADD' });
store.dispatch({ type: 'ADD' });
store.dispatch({ type: 'ADD' });
store.dispatch({ type: 'SUBSTRACT' });