Solving the toEqual Failing Test Case in Angular Unit Testing

Описание: Discover why the Angular unit test fails when using `toEqual` for comparing arrays of objects and how to resolve the issue effectively.
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This video is based on the question https://stackoverflow.com/q/67933708/ asked by the user 'BreadcrumbPie' ( https://stackoverflow.com/u/11773756/ ) and on the answer https://stackoverflow.com/a/67935077/ provided by the user 'AkshAy Agrawal' ( https://stackoverflow.com/u/10819642/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Understanding Angular Unit Testing: The toEqual Conundrum

When working with Angular, unit testing is crucial for ensuring your components behave as expected. However, you might encounter frustrating moments, especially when testing object equality. A common pitfall involves the use of the toEqual matcher when comparing arrays of objects, which can lead to unexpected failures. This guide delves into the problem and offers a clear solution to your testing woes.

The Problem: Getting Unexpected Test Results

You may find yourself in a situation where you have set up your unit tests correctly, but they still fail. For instance, consider the following scenario involving employee data in an Angular component:

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This code snippet populates an array of employee objects based on the input provided. The issue arises during unit testing, where an expectation fails:

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The expectation using toEqual fails, raising errors about object types not being equal, despite appearing to contain the same information.

The Solution: Why toEqual Fails and How to Fix It

Understanding toEqual in Angular Testing

The crux of the issue lies in how toEqual evaluates objects. It does not simply compare the values of the properties; it also checks that the objects being compared are the same instance in memory. Since the objects in mockEmpOut and component.testVar are created at different times, they reside in different memory locations, leading to the comparison failing.

Why Does This Happen?

When you create mockEmpOut with new objects, it's generating a different instance in memory.

When you call convertEmpArr(), new instances of Employee objects are created, resulting in a different memory reference.

Workarounds

Method 1: Use JSON.stringify()

As you noted in your test, you can use JSON.stringify() to serialize the objects before making a comparison. This method converts the objects into strings, allowing for a direct comparison, albeit at the cost of performance.

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Method 2: Comparing Object Properties Directly

Another more efficient way is to assert the individual properties of the objects instead of comparing the whole object at once. Here’s how you can implement this:

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Conclusion: Testing with Confidence

Understanding why certain testing assertions fail can significantly improve your development process. By grasping the behavior of toEqual, you can adopt better strategies for comparing complex data types. Whether you opt for the JSON.stringify() approach or check property values directly, knowing how to work around these limitations will help you create robust unit tests for your Angular applications.

With these insights, you can proceed with your Angular unit testing confidently, ensuring that your components function correctly as intended. Happy coding!