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Discussion on: Don't duplicate your data - Learnings from code reviews

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Johannes Kettmann Author

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 :)