Solving the Cannot read properties of undefined Error in React Query

Learn how to troubleshoot and fix the `Cannot read properties of undefined` error in React Query, especially when handling parameters with a deep dive into best practices and essential code adjustments.
---

Visit these links for original content and any more details, such as alternate solutions, latest updates/developments on topic, comments, revision history etc. For example, the original title of the Question was: React-query returns Cannot read properties of undefined even though I can see id is set

If anything seems off to you, please feel free to write me at vlogize [AT] gmail [DOT] com.
---
Solving the Cannot read properties of undefined Error in React Query

If you've been working with React Query and encountered the frustrating error message "Cannot read properties of undefined" even though your console log shows the correct value, you're not alone. This is a common pitfall for developers using React Query, especially when dealing with parameters. In this guide, we'll analyze the problem and guide you through the solution step-by-step.

Understanding the Error

The error usually arises when trying to access a property of an object that hasn’t been defined yet. In this case, it manifests during data fetching when React Query attempts to retrieve data from an API, but the input parameters for that fetch are not structured correctly.

Common Scenario with React Query

Your Code

Consider the following code snippet that is attempting to fetch data based on an id:

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

Here, React Query is set up to query data based on id but it’s throwing an error regarding obj being undefined in the fetching function.

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

The Problem

In the above example, the issue arises because the second argument (obj) in the GetList function is undefined. Although logging idTest reveals it has a value, React Query needs to access the parameters correctly to structure the request properly.

Steps to Solve the Issue

1. Adjust the Query Function

Since version 3 of React Query and beyond, query functions are passed a QueryFunctionContext, including the queryKey array containing your parameters. You need to unpack these correctly in your GetList function.

2. Update the GetList Function

Change your GetList function to accept queryFunctionContext and extract the parameters from the queryKey array as follows:

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

3. Verify QueryKey Structure

Ensure your useQuery is structured correctly so that it sends the expected key:

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

4. Error Handling

Continue to keep your error handling robust to address cases where data may still not be as expected:

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

Conclusion

By restructuring your query function to properly accept and utilize the QueryFunctionContext, you can effectively resolve the "Cannot read properties of undefined" error. Ensure you always unpack your parameters correctly, especially when leveraging external libraries like React Query for data fetching.

With these fixes and practices, managing data in your React applications should be much smoother. Happy coding!