Скачать или смотреть How to Fix the Password Comparison Issue in Angular Template Driven Forms

Learn how to resolve the issue of an undefined `@ input` value when comparing passwords in Angular Template Driven Forms by using the correct Azure dependency injection strategy.
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This video is based on the question https://stackoverflow.com/q/63262028/ asked by the user 'Swarup Chavan' ( https://stackoverflow.com/u/5193594/ ) and on the answer https://stackoverflow.com/a/63262356/ provided by the user 'yurzui' ( https://stackoverflow.com/u/5485167/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Troubleshooting Password Comparison in Angular's Template Driven Forms

When working with Angular's Template Driven Forms, you may encounter a challenge: trying to compare two password fields, but finding that the @ Input value is always undefined in the validation function. This problem can be frustrating, especially if you've followed several examples online without success. Let's break down why this occurs and how to resolve it effectively.

Understanding the Problem

In Angular, when you create a directive to compare passwords—say a password and its confirmation—you expect the input value to be correctly tied to your form controls. However, if you set up your directive incorrectly, you might see unexpected behavior including:

The password doesn't validate as expected.

Console logs displaying undefined for the comparison input.

In this guide, we'll explore the solution to this issue step by step.

The Code Breakdown

Template Code

Your template for the password comparison might look something like this:

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

Directive Code

Your directive to perform the validation might look like this:

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

The Issue Identified

The issue arises from how Angular creates instances of your directive. In your current setup, you're using the useClass property in the providers array to create a new instance of the ValidateEqualDirective specifically for validation purposes. Consequently, this creates two instances of your directive:

One instance tied to your HTML structure.

Another new instance created exclusively for validation.

This is why you're seeing undefined for this.controlToCompare in your logs—you are referencing the wrong instance!

The Solution

To fix this issue, you need to leverage the existing instance of your directive instead of creating a new one. Here's how to adjust the provider configuration:

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

By changing useClass to useExisting, you're instructing Angular to reuse the current instance of the directive. This should allow the @ Input value to be correctly set and accessible in both the constructor and the validation function.

Key Takeaways

Directive Instance: Use useExisting to refer to the already created instance of the directive.

Validation Logic: Make sure your validation logic accesses the correct instance tied to the form controls.

Conclusion

By understanding the relationship between your Angular directives and how they are instantiated, you can effectively solve the problem of undefined @ Input values in password comparison validation. Implement the adjustment to your provider setup, and you should see everything functioning as expected!

Feel free to reach out if you have any questions or need further assistance. Happy coding!