bookSubjectModifierSelectors.js

const modifyingBooksSelector = createSelector(
    [state => state.booksSubjectsModifier.singleBookModify, state => state.booksSubjectsModifier.selectedBooksModify, state => state.books],
    (singleBookModify, selectedBooksModify, books) => {
        let modifyingBookIds = singleBookModify ? [singleBookModify] : (selectedBooksModify ? Object.keys(books.selectedBooks) : []);
        return modifyingBookIds.filter(_id => _id).map(_id => books.booksHash[_id]);
    }
);

const addingSubjectsSelector = createSelector(
    [state => state.booksSubjectsModifier.addingSubjects, state => state.subjects.list],
    (adding, subjects) => Object.keys(adding).filter(_id => adding[_id]).map(_id => subjects[_id])
);

const removingSubjectsSelector = createSelector(
    [state => state.booksSubjectsModifier.removingSubjects, state => state.subjects.list],
    (removing, subjects) => Object.keys(removing).filter(_id => removing[_id]).map(_id => subjects[_id])
);

const booksSubjectsModifierSelector = createSelector(
    [state => state.booksSubjectsModifier.addingSubjects, state => state.booksSubjectsModifier.removingSubjects,
        modifyingBooksSelector, addingSubjectsSelector, removingSubjectsSelector],
    (addingSubjectIds, removingSubjectIds, modifyingBooks, addingSubjects, removingSubjects) => ({
        addingSubjectIds,
        removingSubjectIds,
        modifyingBooks,
        addingSubjects,
        removingSubjects
    })
);