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Learn how to implement `optimistic updates` in your React app using React-Query. Discover the common pitfalls, particularly around state management, and how to effectively handle them.
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This video is based on the question https://stackoverflow.com/q/71128489/ asked by the user 'Vladimir Kosenko' ( https://stackoverflow.com/u/7208772/ ) and on the answer https://stackoverflow.com/a/71133697/ provided by the user 'anonkey' ( https://stackoverflow.com/u/8403915/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Mastering Optimistic Updates with React-Query: A Guide for Beginners

In the world of web applications, user experience often hinges on the responsiveness of the interface. One popular technique developers use to create a seamless experience is optimistic updates. This allows the application to predict changes in state and reflect them in the interface before the server confirms them. However, this approach can be tricky, especially when dealing with libraries like React-Query. Let's dive into a common issue you may encounter and how to solve it effectively.

The Problem

You've found yourself trying to implement an optimistic update using the mutation function in React-Query but hit a bump in the road. Specifically, you're unable to fetch the previous value from a query properly using queryClient.getQueryData(), and your modifications don't seem to work as expected.

For instance, you may find that after updating a post or comment, the previousCurrentPost always returns the value that has already been updated. This can be frustrating, especially for those new to working with React-Query.

Understanding Your Code

Here's a simplified version of your onMutate function where the problem arises:

[[See Video to Reveal this Text or Code Snippet]]

The Solution

The issue you've encountered stems from how JavaScript manages objects. In your case, the line:

[[See Video to Reveal this Text or Code Snippet]]

creates a shallow copy of oldPost. This means that while updated is a new object, it still references the same objects for nested structures (like oldPost.state and updated.state). As a result, when you modify properties of updated.state, you're also modifying oldPost.state, which is not what you intended.

Making a Deep Copy

To resolve this, you need to ensure that you are creating a deep copy of your state. Here are a couple of ways to approach this:

Using Structured Clone (supported in most modern browsers):

[[See Video to Reveal this Text or Code Snippet]]

This method creates a deep copy of the object without modifying the original.

Manual Deep Copy:
If you're looking for a solution that works in all environments or requires more control, consider manually creating a deep copy using utility functions like JSON.parse(JSON.stringify(object)) (note this has limitations with functions and circular references).

Adjusting Your Code

With this change, your onMutate function could look like this:

[[See Video to Reveal this Text or Code Snippet]]

Conclusion

Optimistic updates can greatly enhance the user experience in your applications, but they require careful management of state. By understanding the difference between shallow and deep copying of objects, you can avoid common pitfalls and ensure your application's state remains consistent and reliable. Hopefully, this breakdown helps you confidently implement optimistic updates in your projects with React-Query!

Now that you're armed with this knowledge, leap forward and make your app feel more responsive to the user's actions!