Скачать или смотреть Resolving Number.isInteger Overwrite Issues in Jest Tests

Learn how to successfully overwrite the built-in `Number.isInteger` method in Jest tests, ensuring effective testing of your polyfills.
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This video is based on the question https://stackoverflow.com/q/64461886/ asked by the user 'Michał Tkaczyk' ( https://stackoverflow.com/u/6169436/ ) and on the answer https://stackoverflow.com/a/64462078/ provided by the user 'Estus Flask' ( https://stackoverflow.com/u/3731501/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Overwriting Number.isInteger in Jest: A Common Pitfall and How to Fix It

When working with JavaScript, especially in testing environments like Jest, developers often encounter scenarios where they need to overwrite built-in methods for testing purposes. One common issue arises when trying to test custom polyfills, such as a reimplementation of the Number.isInteger function. In this post, we will delve into the problem of overwriting the Number.isInteger method in your Jest tests and provide a structured solution to ensure your polyfill can be properly tested.

The Problem

You have created a polyfill for the built-in Number.isInteger function, but when you're trying to test this polyfill using Jest, the overwriting of Number.isInteger does not behave as expected. This can lead to situations where your tests pass, but they do not accurately test your custom implementation, since the method was only being evaluated at the time of import.

Here’s the essence of the test you’re trying to run:

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While the test might pass, it does not invoke the second part of your polyfill definition. Thus, you're left wondering how to correctly overwrite this method to validate the functionality of your polyfill.

The Solution: Step-by-Step

To address the issue of properly testing your Number.isInteger polyfill, follow these structured steps:

1. Store the Original Method

Before making any modifications, it's crucial to keep a reference to the original Number.isInteger. This way, you can restore it after your test, ensuring that your changes do not affect other tests or the global environment.

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2. Restore After Each Test

Using afterEach, you can ensure that Number.isInteger is always reset to its original method after each test case runs. This preserves the integrity of your testing environment.

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3. Use Jest's isolateModules API

To properly test your polyfill, you need to isolate the module where isInteger is defined. This allows you to mock Number.isInteger without it being evaluated prematurely. Here's how you can do it:

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4. Note on Method Existence

Remember, Number.prototype.isInteger does not exist and does not need to be mocked. Instead, focus on testing your polyfill as a static method.

Conclusion

By following the steps outlined above, you can effectively overwrite the built-in Number.isInteger method in your Jest tests, allowing you to accurately assess the functionality of your polyfill. Always keep your tests isolated and reset any modifications afterward to maintain a clean testing environment. This practice will help you avoid common pitfalls and ensure your testing suite remains reliable.

With this approach, you can confidently validate that your custom polyfills work as intended, enhancing the robustness of your JavaScript applications.