Simple redux example

store.js

const {createStore} from 'redux';

// this is how state looks initially
const INITIAL_STATE = {
  counter: 0
};

function reducer(state=INITIAL_STATE, action) {
  // this function will return a new state
  // based on action
  switch (action.type) {
    case 'INCREMENT':
      return {...state, counter: state.counter + 1};
    case 'DECREMENT': 
      return {...state, counter: state.counter - 1};
    default:
      return state;
  }
}

// createStore function is exported from redux and is used
// to create a redux store
const store = createStore(reducer);

store.dispatch({type: 'INCREMENT'}); // counter = 1 now
store.dispatch({type: 'INCREMENT'}); // counter = 2 now
store.dispatch({type: 'DECREMENT'}); // counter = 1 now