Скачать или смотреть How to Test ngFor with Jest in Angular 14

Discover how to effectively test the `ngFor` directive in your Angular 14 components using Jest, focusing on accessing dynamic HTML elements generated in loops.
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How to Test ngFor with Jest in Angular 14: A Complete Guide

Testing components in Angular is essential for ensuring that your application runs as expected. In this guide, we're diving into a common challenge faced by developers: how to test an ngFor directive using Jest. If you've ever struggled to retrieve elements inside an ngFor loop and access their attributes during a test, this guide is for you.

Understanding the Problem

When working with a breadcrumbs component in Angular, you need to ensure that the dynamic elements generated by ngFor are being rendered correctly. Let's break down our situation:

You're trying to test a breadcrumbs component that iterates over an array of breadcrumb objects.

You want to retrieve an <a> element and verify its routerLink attribute, but your queries return null.

You suspect that this issue is related to change detection within Angular's lifecycle.

Code Snippet: The Component

Here's a simplified version of the component we're working with:

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

In this component:

An <ng-container> with *ngFor generates list items based on breadcrumbs array.

An <a> tag is conditionally rendered depending on whether the breadcrumb is active.

The Approach to Testing

Setting Up the Component Test

To test this component, we start by setting it up using Angular's testing utilities in Jest. Here's an initial setup for your test file:

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

Writing the Test Case

Now, let's write a test case that checks if we can retrieve the <a> element and its routerLink correctly. Here's how to do it:

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

Explanation of the Solution

Detect Changes: The most critical step in this process is calling fixture.detectChanges(). This ensures that Angular updates the component's view based on the new breadcrumbs property, allowing the <a> tag to be rendered correctly.

Query the DOM: Once you have called detectChanges(), you can successfully query for the <a> element.

Assert the Attribute: Finally, verify if the href attribute of the <a> element matches the expected to value from the breadcrumbs array.

Conclusion

Testing components that utilize ngFor in Angular can present unique challenges, especially when accessing dynamically generated elements. By effectively utilizing the fixture.detectChanges() method, we can ensure our components are tested accurately.

If you're facing difficulties with your Angular testing strategy, remember that understanding the component lifecycle and ensuring updates through change detection is key. Happy coding and testing!