Creating an asynchronous validator function

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, for instance. These situations would require something called asynchronous validators. In this recipe, you're going to create your first asynchronous validator.

Getting ready

The project that we are going to work with resides in chapter08/start_here/asynchronous-validator inside the cloned repository:

1. Open the project in Visual Studio Code.
2. Open the terminal and run npm install to install the dependencies of the project.
3. Once done, run ng serve -o.

   This should open the app in a new browser tab and you should see something like...