Reducer.js

const clear = (favoriteId) => state => ...
const fetching = (favoriteId) => state => ...
const finishFetching = (favoriteId, favorite) => state => ...

export default (state = {}, action) => {
    switch (action.type) {
        case 'CLEAR_FAVORITE':
            return clear(action.favoriteId)(state);
        case 'FETCHING_FAVORITE':
            return fetching(action.favoriteId)(state);
        case 'FINISH_FETCHING_FAVORITE':
            return finishFetching(action.favoriteId, action.favorite)(state);
        default:
            return state;
    }
}