by @tomastrajan from AngularExperts.io

In this exercise, we're going to explore how to implement Angular form validation

• How to implement form validation
• How to use built in and custom validators
• How to add validation on various levels (eg, form control vs from array)
• How to display validation errors
• How to display form validation errors on submit
• How to remove validation errors with reset

Compared to where we left off we have added a number.validator.ts file in the core/validator/ folder with a validator stub in order to provide TODO items for the exercise

Search for the TODO <index>:, eg TODO 1: items in the code itself by searching the projects folder using IDE or text editor search functionality

Before we get started, it can be a good idea to adjust Eslint IDE settings, especially the Working directories to match the current exercise to prevent false positive errors and warnings.

• The "contract" between the required and every other validator
• Why do we have to "call" () some validators and not others?
• Why do we always want to define custom validators as a factory function? (vs a property similar to Validators.required)
• What happens if you use type="submit" button and then try to reset() the form?

• every exercise folder should be installed using npm ci
• every exercise can and should be started using npm start to run the app
• every exercise can start tests in watch mode using npm run test:watch
• single test can be run with npm t -- <part-of-the-test-file-name>, eg npm t -- product-list
• every exercise has its own README.md file with additional description of the given exercise
• every exercise project contains ordered TODO in the readme and // TODO comments inside the source code (eg // TODO 2: description) which should be followed to complete the given exercise
• you can always search for // TODO, or <!-- TODO or check README.md for the next TODO item