When using Reactive Forms in Angular, it’s very easy to define custom validators, as they are nothing more than regular functions. You could create the function for your custom validator inside your component file directly if the validator won’t be used elsewhere, but here we’ll assume re-use and create a validator in a separate file. Form Validation. AngularJS offers client-side form validation. AngularJS monitors the state of the form and input fields (input, textarea, select), and lets you notify the user about the current state. AngularJS also holds information about whether they have been touched, or modified, or not. Jun 13, 2017 · On a recent Angular 4 project I had the need to use some custom form validations. Out of the box, Angular 4 contains validators for required, email, pattern, and min/max length, and it is possible to write your own validators as well. In particular, the custom validation I needed to do was verify that two fields were equal.