redux.js

// default state
const defaultState = {
  welcome: 'Hello',
  otherState: 'some stuff',
}

// reducer, always returns new object based on the action type
const greeting = (state = defaultState, action) => {
  switch (action.type) {
    case 'GREET_ME':
      return {
        ...state,
        welcome: 'Hello Antonio',
      }
    case 'GREET_WORLD':
      return {
        ...state,
        welcome: 'Hello World',
      }
    default:
      return state
  }
}

// creates store from reducers
const store = createStore(greeting)

console.log(store.getState())
// {welcome: "Hello", otherState: "some stuff"}

// action is an object describing what we are dispatching
store.dispatch({
  type: 'GREET_ME'
})

console.log(store.getState())
// {welcome: "Hello Antonio", otherState: "some stuff"}