Скачать или смотреть How to Create an Instance of a Class with Mock Services in Angular Testing

Discover the best practices for creating class instances with mock services in Angular 9 unit testing. Learn how to effectively utilize TypeScript features to overcome challenges while testing your Angular applications.
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Creating Class Instances with Mock Services in Angular Testing

Unit testing is an essential part of Angular development, helping to ensure that your components and services work as expected. However, when it comes to testing non-component classes, such as data sources for Angular Material Trees, developers often encounter various challenges. In this guide, we'll focus on a specific issue: creating instances of a class with mock services during Angular testing and how to address it effectively.

Understanding the Problem

In an Angular 9 project, you may have created a class called DynamicDataSource, which acts as the data source for a Material Tree in another component. If your experience lies primarily in testing components rather than classes, you might face difficulties in instantiating your class within a test file.

The Key Issues

Error Messages: Typically, instantiating your class directly while trying to use a mock service leads to errors, such as:

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Private Properties: Attempting to define properties in your mock service can be problematic if they are private.

Testing Methods: You want to ensure that the methods within your class can be tested properly.

Step-by-Step Solution

Let’s break down the solution to address these challenges effectively.

1. Using Type Assertion

The simplest way to create an instance of DynamicDataSource with the mock service is to utilize Type Assertion to override TypeScript's checking. Instead of directly passing the mock service, you can do the following:

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Why This Works

The statement above tells TypeScript to treat mockMyService as any type, bypassing the strict type checks. This allows you to instantiate your class successfully.

2. Stubbing Required Properties

Even after overcoming the TypeScript issue, you need to ensure that your mock service mimics the actual service's behavior to avoid runtime errors. This means implementing mock methods and properties within your MockMyService class.

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By defining these methods as spies, you can effectively track their calls and behaviors within your tests.

3. Writing Your Test Cases

With the above steps in place, you can now write a test case to verify that your DynamicDataSource class is instantiated properly and behaves as expected.

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Conclusion

Unit testing non-component classes in Angular, like DynamicDataSource, can present obstacles due to strict type enforcement in TypeScript and the need to mock complex services. However, by using Type Assertion and effectively stubbing the required properties and methods of the service, you can easily create test instances and ensure your classes operate correctly.

With this approach, you'll be equipped to enhance the reliability of your Angular applications while mastering unit testing strategies. Happy testing!