const initialState = {
    categories: []
}


case "FETCHING_SUCCESS":
      return {
        ...state,
        isAdding: false,
        categories: [...state.categories, action.data]
      }

[
 {id:1, name: 'name01', someId: 11},
 {id:2, name: 'name02', someId: 22},
 {id:3, name: 'name03', someId: 33}
]

case "FETCHING_SUCCESS":
      return {
        ...state,
        isAdding: false,
        categories: action.data     // <= Notice This
      }