Скачать или смотреть Fixing useQuery Conditional Rendering Issues in React

Learn how to eliminate the `length undefined` error with `useQuery` in React components, ensuring smooth data rendering.
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This video is based on the question https://stackoverflow.com/q/64528101/ asked by the user 'SmithBWare89' ( https://stackoverflow.com/u/13443220/ ) and on the answer https://stackoverflow.com/a/64528521/ provided by the user 'Wael Zoaiter' ( https://stackoverflow.com/u/10961055/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Fixing useQuery Conditional Rendering Issues in React

If you've ever encountered the dreaded length undefined error while trying to render data fetched using useQuery, you're not alone. Many developers face this issue while working with React and Apollo, especially when dealing with asynchronous data. This guide will guide you through the problem and offer a straightforward solution to make your data rendering both reliable and efficient.

Understanding the Problem

When using the useQuery hook from Apollo Client to fetch data, it can sometimes lead to scenarios where the component attempts to access properties of an object that hasn't been fully initialized yet. This issue typically arises when the data being fetched is still loading, or if certain properties within that data might not exist during the first rendering.

In our case, the error occurs when trying to access the length of an array in userData?.savedBooks.length, especially when the data hasn't yet populated correctly. This can lead to a frustrating experience, causing the entire rendering to fail. The primary culprit here is that savedBooks can be undefined at various points, especially while data is being fetched.

The Solution

To overcome this issue, follow these steps to make sure your component can handle the data correctly without throwing errors.

1. Defining a Safe Check for Undefined

We'll add an additional optional chaining operator to our check when rendering the savedBooks. This helps ensure that we don't try to access properties on undefined values.

Change this line in your JSX:

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

To this:

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

2. Using useEffect for Data Updates

Additionally, another common pitfall is directly calling setUserData within the body of your functional component. Doing so can lead to unnecessary re-renders or even an infinite render loop. Instead, encapsulate your setUserData logic within a useEffect hook which will only run when the data changes.

Here's how you can implement it:

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

This way, setUserData will only be executed when your data changes, preventing unnecessary updates and ensuring that your component remains efficient.

Putting It All Together

Now, your complete SavedBooks component should look something like this:

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

Conclusion

By implementing these techniques, you can prevent the frustrating length undefined error in your React components when using useQuery from Apollo. Not only will this improve the reliability of your UI, but it also enhances the overall user experience by providing quick feedback when data is still being loaded. Happy coding!