blog-async-await-callbacks.js

function addComment(comment, done) {
  fetchPostBySlug(comment.postSlug, (err, post) => {
    if (err) return done(err);

    if (!post.isCommentingEnabled) {
      return done(new Error('Commenting is disabled.'));
    }

    isUserAllowedToComment(post, comment.user, (err, isAllowed) => {
      if (err) return done(err);

      if (!isAllowed) {
        return done(
          new Error('The user is not allowed to comment on this post.')
        );
      }

      insertComment(post.id, comment.user, comment.message, (err, result) => {
        if (err) return done(err);

        return done(null, result.record);
      });
    });
  });
}