Resolving NullInjectorError in Angular 17 with Functional Guards

Discover how to fix the `ERROR NullInjectorError` while migrating to Angular 17 by implementing functional guards correctly. This guide walks you through the essential steps to ensure service providers are configured properly.
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Solving NullInjectorError in Angular 17 with Functional Guards

Migrating an Angular project to a new version can often lead to various issues, especially when it involves changes in code structure and patterns. One common problem developers face in Angular 17 is the NullInjectorError, particularly when working with functional guards. If you're encountering the error message stating "No provider for _UserService!", you're not alone. Let’s dive into the root cause of this issue and how to effectively resolve it.

Understanding the Problem

When you switched from class-based guards to functional guards in Angular 17, you might have expected everything to work seamlessly, but instead you are greeted with the following error:

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

This error typically indicates that Angular's dependency injection system is unable to find a provider for UserService. This can occur when a service is not registered in the application's providers, even if you're using the inject function to access it.

Analyzing the Guard Code

Here's a look at your authGuard implementation:

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

What's Happening Here?

In this code, you're using the inject method to retrieve instances of UserService and Router. While this is indeed a valid approach in functional guards, the critical part to note is that Angular requires services to be provided in order to resolve dependencies correctly.

The Root Cause

Implementing the Solution

To resolve this error, follow these simple steps:

Modify the Providers Array: Add your UserService and any other necessary services to the providers list in your config. Your updated configuration will look like this:

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

Save Your Changes: After updating the configuration file, save your changes and restart your Angular application.

Conclusion

By adding UserService to the providers array in your application configuration, you allow Angular’s dependency injector to correctly provide an instance of UserService wherever it is needed, including in your functional guards. Always remember to register services that are crucial for your business logic, especially when migrating to newer Angular versions.

These adjustments will help you avoid NullInjectorError while utilizing the benefits of functional guards in Angular 17. Happy coding!