Скачать или смотреть # How to Resolve the TypeError: Cannot set property 'birthday' of undefined in Angular with Jest

Learn how to solve the common error `Cannot set property 'birthday' of undefined` when writing unit tests for Angular components using Jest. Follow our step-by-step guide to fix the mistake and ensure your tests run smoothly.
---
This video is based on the question https://stackoverflow.com/q/63234088/ asked by the user 'Carolina' ( https://stackoverflow.com/u/8678068/ ) and on the answer https://stackoverflow.com/a/63234205/ provided by the user 'micronyks' ( https://stackoverflow.com/u/3751711/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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: Jest: Cannot set property ' ' of undefined (Angular)

Also, Content (except music) licensed under CC BY-SA https://meta.stackexchange.com/help/l...
The original Question post is licensed under the 'CC BY-SA 4.0' ( https://creativecommons.org/licenses/... ) license, and the original Answer post is licensed under the 'CC BY-SA 4.0' ( https://creativecommons.org/licenses/... ) license.

If anything seems off to you, please feel free to write me at vlogize [AT] gmail [DOT] com.
---
Understanding the TypeError in Jest with Angular

When writing unit tests for Angular components, you may encounter various error messages that can be perplexing, especially when using Jest. One such error is TypeError: Cannot set property 'birthday' of undefined. This error typically indicates that the component is trying to access a property of an undefined object. In this guide, we'll break down this issue and guide you through a solution in a clear and structured manner.

The Problem

The error occurs in this specific case due to a missing input property in the test setup. Let's go through the relevant portion of the code to understand the context of the issue.

In CandidateItemComponent.ts, the birthday value is fetched from the candidate object:

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

If the candidate input is not properly initialized before the component lifecycle runs, trying to set this.birthday will lead to a TypeError since this.candidate is undefined.

The Solution

To resolve this issue, you need to ensure that the candidate property is set in the unit tests before invoking fixture.detectChanges(). Here’s how to fix the test code in CandidateItemComponent.spec.ts:

Step-by-Step Fix

Initialize the Input Property: Add the initialization of the candidate property to your unit test setup.

Update the Test File: Modify the test to set the candidate property before you run fixture.detectChanges().

Here’s the updated test code:

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

Key Takeaways

Always initialize all inputs: When writing tests for Angular components, ensure that you have initialized all necessary input properties that the component depends on.

Check component lifecycle: Pay attention to the component’s lifecycle methods, such as ngOnInit, which access properties that need to be defined.

Use Mock Data: Utilize mock data to simulate the real input for your tests. This helps to ensure your tests are robust and accurately reflect potential use cases.

Conclusion

By following the outlined steps, you can resolve the TypeError: Cannot set property 'birthday' of undefined when working with Angular and Jest. Proper input property initialization is crucial to successfully testing your components without running into common pitfalls. Happy coding, and may your unit tests run smoothly!