redux/src/reducer.js

const initialState = {
    counter: 0,    
  }
  
const reducer = (state = initialState, action) => {
    switch (action.type){
    case 'INC':  
      return {...state, 
        counter:
          state.counter + 1
        };
    case 'DEC':
     return {...state, 
      counter:
        state.counter - 1
      };
    case 'RND':
      return {...state, 
        counter: state.counter * action.payload
      };
    default:
      return state
    }
  };

 export default reducer;