How to Fix State Issues in Custom Hook Tests with ReactJS and JestJS

Learn how to resolve issues with custom hooks where state does not update as expected in `ReactJS` tests using `JestJS` and `React Testing Library`.
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This video is based on the question https://stackoverflow.com/q/65696581/ asked by the user 'Satoru Kikuchi' ( https://stackoverflow.com/u/10760140/ ) and on the answer https://stackoverflow.com/a/65698967/ provided by the user 'darkscripter' ( https://stackoverflow.com/u/13065945/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Understanding State Issues in Custom Hook Tests

When you're working with custom hooks in React, testing their behavior is crucial to ensure your application functions as expected. However, you might run into problems where the state does not change from its initial value. This post discusses one such scenario and provides a clear solution.

The Problem

Imagine you're testing a custom hook called useCount, which is supposed to manage a numerical count. Here’s a quick look at the hook’s code:

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

In your test code, you attempt to reset the count to zero and then call the increment function:

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

However, you find that the test fails because count remains at its initial value of 100, and you may wonder why this is happening.

The Explanation

The issue arises because you're attempting to change the state (setCount(0)) and invoke the increment function within the same render cycle. As a result, the final value of count does not reflect your changes. After setting the count to zero, the increment logic is still using the initial state of 100.

Key Points to Consider:

Render Cycle: The state updates and function calls occur in a single render cycle, causing the hook to return the initial state.

State Persistence: Each change to state needs to be isolated in its render cycle to see the new values successfully reflected.

The Solution

To fix this issue, you need to ensure that after you call setCount, you trigger a re-render before calling increment. This way, the increment function will operate on the updated state rather than the initial state. Below is the modified test code:

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

Steps Summary:

Set the Count: Call setCount to change the initial value.

Rerender: Manually trigger a re-render using rerender().

Increment the Count: Then, call the increment function.

Conclusion

Testing custom hooks can sometimes lead to unexpected results if you're not careful about how React handles state updates and renders. By ensuring that you separate the state changes and re-renders as shown, you can successfully test the state changes in your custom hooks. Happy coding!

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