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. For example, a form where the user enters a phone number, and then confirms the phone number in a second form field. The custom validation would verify that the two text inputs had the same value.