herdulikecomposition.js

function itemsReducer(state, action) {
  switch(action.type) {
    case 'UPDATE_ITEM':
      return state.items.map(i => itemReducer(i, action))
    default:
      return state
  }
}

function itemReducer(state, action) {
  switch(action.type) {
    case 'UPDATE_ITEM':
      return {
        ...state,
        [action.payload.fieldToChange]: action.payload.newValue
      }
    default:
      return state
  }  
}