Don't duplicate your data - Learnings from code reviews

Handling data properly can be difficult. We have to fetch it from APIs. We have to aggregate it with data from other sources. And we have to transform it efficiently to use in our UIs.

In the past months, I conducted many code reviews for junior developers during this new course. I was surprised to see one mistake over and over again. A mistake that can lead to nasty bugs that are very hard to debug.

This post is about data duplication and its antidote: a single source of truth.

Before I explain what that means let's have a look at a code example.

Duplicated data

The following component renders a list of blog posts that it receives from its parent.

A user can select a filter to show only posts that were created on a particular day. The component filters and renders the provided posts accordingly.

const PostList = ({ posts }) => {
  const [selectedDay, setSelectedDay] = useState(null);
  const [filteredPosts, setFilteredPosts] = useState(posts);

  const onChangeDay = (day) => {
    setSelectedDay(day);
    const postsForDay = posts.filter(
      (post) => isSameDay(post.createdAt, day)
    );
    setFilteredPosts(postsForDay);
  };

  return (
    <Wrapper>
      <Filter
        selectedDay={selectedDay}
        onChangeDay={onChangeDay}
      />
      {
        filteredPosts.map((post) => (
          <Post key={post.id} {...post} />
        ))
      }
    </Wrapper>
  );
};

To implement the filtering the selected day is stored in a state variable. Next to the selected day, we find another state variable that holds the filtered posts.

This filteredPosts array is then rendered below. It is updated whenever the selected day changes inside the onChangeDay callback.

Maybe you realized the problem with this approach: the filteredPosts state is just a subset of the posts prop. We duplicate part of the posts array and thus store the data in two different places.

Ok, true.

But what's the problem here?

We have to keep the duplicates in sync with the originals.

Imagine the following situation: The parent component allows the user to edit a post. The user decides to change the title of a post from "Data duplication rocks!" to "Data duplication sucks!".

What would happen now?

1. The parent component re-renders with the updated posts array.
2. The PostList component re-renders with the updated posts prop.

So far so good. But remember how the component looks like:

const PostList = ({ posts }) => {
  const [selectedDay, setSelectedDay] = useState(null);
  const [filteredPosts, setFilteredPosts] = useState(posts);

  const onChangeDay = (day) => { ... };

  return (
    <Wrapper>
      <Filter ... />
      {
        filteredPosts.map((post) => (
          <Post key={post.id} {...post} />
        ))
      }
    </Wrapper>
  );
};

The PostList actually displays the data from the filteredPosts array. And this a subset of the old version of the posts prop.

That means that the UI would still display the old post with its outdated title "Data duplication rocks!"

The problem is that we only update one version of the post. Our filteredPosts array is out of sync.

A single source of truth

How would a better version of our component look like?

We wouldn't copy the data into another state variable. We would try to use only one source: the posts prop. A single source of truth.

function PostList({ posts }) {
  const [selectedDay, setSelectedDay] = useState(null);
  const filteredPosts = posts.filter(
    (post) => isSameDay(post.createdAt, selectedDay)
  );

  return (
    <Wrapper>
      <Filter
        selectedDay={selectedDay}
        onChangeDay={setSelectedDay}
      />
      {
        filteredPosts.map((post) => (
          <Post key={post.id} {...post} />
        ))
      }
    </Wrapper>
  );
}

See how we were able to get rid of the filteredPosts state and replace it with a normal variable?

This version is simpler and less likely to introduce a bug.

In case you're worried about performance implications you might be right. If the posts array is very long or the filtering complicated the app might be slow.

But in that case, we could simply make use of the useMemo hook.

const filteredPosts = useMemo(() => posts.filter(
  (post) => isSameDay(post.createdAt, selectedDay)
), [posts, selectedDay]);

The useMemo hook returns a memoized value. The provided function is only run when the dependencies change.

This means that the filtering in the above example is only run when the posts array changes. If the component is re-rendered but the posts array stays the same useMemo simply returns the memoized value and doesn't need to execute the expensive filtering logic again.

Exercise time

Here is another example that could benefit from some simplification.

function Books() {
  const [data, setData] = useState(null);
  const [books, setBooks] = useState([]);

  useEffect(() => {
    fetchData().then((data) => setData(data));
  }, []);

  useEffect(() => {
    if (!data) {
      return;
    }

    const mappedBooks = mapBooks(data);
    setBooks(mappedBooks);
  }, [data]);

  return (
    <div>
      {
        books.map((post) => (
          <div key={post.id}>{post.title}</div>
        ))
      }
    </div>
  );
}

I leave it to you as an exercise to find the problem and refactor this component to use a single source of truth.

Get the refactored code and an explanation of the component above

Follow the link above and drop your email to get my solution to the exercise. I'll also walk you through the original code and explain in detail what it does.

Comments

[Lester016]

When the posts props change that would cause the PostList component to re render so when that components re render all the code inside that component would execute again thus filtering the updated value of props posts. Why it would still filter the old value of props posts? I don't get it.

[Johannes Kettmann]

You're right about the re-rendering part. But state is only initialised when the component mounts.

So during the very first render the selectedDay state is initialized to null and the filteredPosts state is initialized to posts.

Next let's assume that the user selects a day. Then both states are set according to the user selection.

If the parent triggers a re-render of the PostList component for any reason both states variables stay the same. They are not initialized again. That's why the filteredPosts state still contains an old version.

If the component would unmount completely than you would be right. In that case all the code is executed again and the state is initialized to the values I mentioned above.

I hope I could clarify the problem. If you have further questions don't hesitate to ask :)

[ChrisDrit]

Thanks @jkettmann keeping in mind a single source of truth was helpful.

This may be a bit out of scope, but you mentioned the useMemo hook when an:

array is very long or the filtering complicated

could you briefly talk about why that helps? Thanks!

[Johannes Kettmann]

Thanks Chris. That was indeed very short. I added a couple of sentences in the post. Hope that helps :)

[ChrisDrit]

You rock! That was indeed very helpful, thanks!

[John Peters]

Thanks for reminder, also don't duplicate code (ever).