Скачать или смотреть Resolving the Cannot read property 'className' of undefined Error in Angular Tests

Learn how to fix the Angular test error "Cannot read property 'className' of undefined" by using `whenRenderingDone()`. Follow our step-by-step guide for better testing practices.
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Resolving the Cannot read property 'className' of undefined Error in Angular Tests

When running tests in Angular, developers sometimes encounter various errors that can be frustrating and time-consuming to debug. One such error is "Failed: Cannot read property 'className' of undefined". This problem often arises when dealing with components from third-party libraries like Syncfusion, particularly when working with dialogs. In this guide, we’ll explore the reasons behind this error and provide an effective solution to resolve it.

Understanding the Problem

In the given scenario, the spec file (sync.spec.ts) runs several tests successfully, but occasionally, it fails with the className error and another message indicating a timeout on async functions. The issue appears to be connected to the dialog component from Syncfusion, specifically when querying debug elements within the tests. As a result, the error manifests when trying to interact with or check the states of elements that may not be fully rendered yet.

The test in question uses fixtures and waits for elements to stabilize before proceeding. However, even with asynchronous handling in place, sporadic failures occur. Here are the critical aspects of the error:

It occurs primarily due to the asynchronous nature of the rendering process in Angular components, particularly when used with libraries like Syncfusion.

The error message indicates that the code is attempting to access className on an undefined object, implying that the expected element was not found in the DOM.

Solution: Using whenRenderingDone()

The key to resolving this error lies in ensuring that the component's rendering has fully completed before proceeding with any assertions or further interactions. The Angular testing utility provides a straightforward way to handle this by replacing await fixture.whenStable() with await fixture.whenRenderingDone().

Step-by-Step Implementation

Modify the Test Cases: Update your test cases to incorporate the whenRenderingDone() method. In your spec file, here’s how you can make this change:

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

Alternating Calculations and Assertions: Ensure that any calculations or assertions that rely on the DOM being updated should now utilize whenRenderingDone(). For example:

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

Handle Asynchronous Operations with Care: Review all async operations within your components and be certain that they are correctly managed when setting up your test cases.

Conclusion

By replacing await fixture.whenStable() with await fixture.whenRenderingDone(), you ensure that the rendering lifecycle of your Angular components is fully respected during testing. This small but pivotal change can help eliminate the pesky Cannot read property 'className' of undefined error and enhances your overall test reliability.

Feel free to implement this solution in your test suite, and let us know if it resolves your issues! Happy testing!