Experiencing the ResizeObserver is not defined error in Jest can be a common issue when unit testing JavaScript code that utilizes the ResizeObserver API. This error typically arises when running Jest tests on code that depends on this browser-specific feature, which Jest does not natively support. Fortunately, there’s an easy and effective workaround to resolve this problem: mocking the ResizeObserver within your Jest setup.
Why This Error Occurs
The ResizeObserver is not defined error occurs because Jest, a JavaScript testing framework, runs in a Node.js environment where browser-specific objects like ResizeObserver are not available by default. When your code under test uses ResizeObserver, Jest doesn’t recognize it, leading to the error.
The Solution: Mocking ResizeObserver
To bypass this issue, you need to mock ResizeObserver in your Jest environment. Mocking creates a dummy version of ResizeObserver that Jest can understand and work with, allowing your tests to run without encountering the undefined error.
How to Mock ResizeObserver
Here’s the straightforward code to add to your Jest setup file (typically named jest-setup.js or jest.config.js):
// jest-setup.js
global.ResizeObserver = class {
observe() {}
unobserve() {}
disconnect() {}
};
This code snippet defines a mock ResizeObserver class globally within your Jest environment. The mock includes three essential methods: observe, unobserve, and disconnect. These methods are placeholders and do not contain actual logic, but they allow Jest to recognize and interact with ResizeObserver in your tests.
Customizing the Mock
Depending on your specific use case with ResizeObserver and the nature of the tests you’re conducting, you might need to adjust the mock. For instance, if your code relies on certain behaviors or callbacks from ResizeObserver, you may need to expand the mock to accommodate these. Remember, the goal is to replicate the necessary parts of ResizeObserver functionality so that your tests can execute correctly.
Conclusion
By implementing this mock, you can efficiently resolve the ResizeObserver is not defined error in Jest, ensuring your unit tests run smoothly. This approach allows you to test JavaScript code that relies on browser-specific features without any hitches in a Jest testing environment.