Server side validation using asynchronous validator functions

Form validations are pretty straightforward in Angular, the reason being the super-awesome validators that Angular provides out of the box. These validators are synchronous, meaning that as soon as you change the input, the validators kick in and provide you with information about the validity of the values right away. But sometimes, you might rely on some validations from a backend API, or have some asynchronous logic that you need to execute to validate the form value. These situations would require something called asynchronous validators. In this recipe, you're going to create your first asynchronous validator.

Getting ready

The app that we are going to work with resides in start/apps/chapter08/ng-rf-async-validator inside the cloned repository:

1. Open the code repository in your Code Editor.
2. Open the terminal, navigate to the code repository directory and run npm run serve ng-rf-async-validator with-server to serve...