reducer.jsx

const muhReducer = (state, action) => {
  switch(action.type) {
    case 'FROBNICATE':
      return {
        frobnicateCount: state.frobnicateCount + 1,
        ...state,
      }
    default:
      return state
  }
}

// A button

<MuhButton onClick={e => dispatch({ type: 'FROBNICATE' }) } >Frobnicate</MuhButton>

A nested reducer:

const catReducer = (cats, action) => {
  switch(action.type) {
    case 'CAT_BLEND':
      return {
        catLives: cats.catLives - 1,
        ...cats,
      }
    default:
      return cats;
  }
}

const muhReducer = (state, action) => {
  switch(action.type) {
    case 'FROBNICATE':
      return {
        frobnicateCount: state.frobnicateCount + 1,
        ...state,
      }
    default:
      return {
        cats: catReducer(state.cats, action),
        ...state,
      }
  }
}

// A button

<MuhButton onClick={e => dispatch({ type: 'FROBNICATE' }) } >Frobnicate</MuhButton>