How to Use useQuery with useState in React: Troubleshooting Common Issues

Learn how to effectively manage state in React components using Apollo Client's `useQuery` and `useState`. Discover solutions for common issues like state not updating correctly.
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A Guide to Managing State with Apollo Client's useQuery in React

When working with React, managing state can sometimes be tricky, especially when integrating it with data-fetching libraries like Apollo Client. One common issue developers face is the state not updating as expected while using the useQuery hook. In this guide, we will explore this problem and walk through the solution step by step.

The Problem

You have a component that uses Apollo Client's useQuery to fetch data, and you want to update the local state using the setState function in React. However, you're encountering an issue where the state doesn't seem to update when the OnCompleted function is called. Here’s a simplified breakdown of the problem you might face:

Even though the data is fetched correctly and available in the data object, the state does not reflect these updates.

You’re curious whether your table that maps this data will update accordingly.

So, how can we resolve these issues?

The Solution

Fixing the State Update Issue

The first thing you should check is your spelling. The correct prop name to use in the useQuery options is onCompleted, not OnCompleted. The case sensitivity in JavaScript is crucial, and any misspelling will prevent the function from executing correctly, leaving your state undefined.

Here's the corrected line in your code:

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

Understanding State Updates in React

Fetching Data: The useQuery hook retrieves your data from the server.

Updating State: The onCompleted function updates your state with the fetched data.

Re-render: Once the state has changed, React triggers a re-render of the component, causing the table mapping to update according to the new horses state.

Optional: Optimizing Data Handling

For better performance, especially if you're planning to filter the results later, it might not be essential to store the data in local state. Instead, you could do one of the following:

Filter on-the-fly: Filter the horses directly when mapping, which conserves space and can sometimes improve speed.

Use useMemo: Utilize the useMemo hook to memoize the filtered results, reducing unnecessary recalculations and re-renders.

Here’s an example of how to implement filtering:

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

Conclusion

In summary, ensuring that you use the correct property names and understanding the React lifecycle will help you effectively manage your component’s state with the useQuery hook from Apollo Client. As you build more complex applications, keeping these principles in mind will be invaluable.

Key Takeaways:

Check for case sensitivity when it comes to props in React.

Understand how state updates trigger re-renders.

Optimize performance by filtering efficiently.

Now you're equipped to tackle issues related to state management with useQuery and useState in React. Happy coding!