Q:

Preforming checks in redux selectors

Is it normal to preform checks in selectors, when using reselect ?
const getComments = state => state.entities.comments;
const getCommentIds = (state, props) => {
  const id = props.match.params.post;
  const comments = state.pagination.comments[id];

  return (comments && !comments.isFetching) ? comments.ids : []; // this line
}

export const getCommentsForPost = createSelector(
  [ getComments, getCommentIds ], (comments, ids) => {
    return ids.map(id => comments[id]);
  }
)

A:

Selectors are the public interface for the rest of your app to access your store's data. Putting in checks to make your consuming app code nicer is fine in my book.
For the specific case you have above, I might instead just pass in the isFetching boolean value down to the component as well and let it handle how to handle what it displays when fetching and not, but what you have is totally fine if it gets the behavior you are looking for.

