Custom validation

In the previous section, we saw how to use validators and combine validators together to create more complex validation. The Validators.required, Validators.minLength, Validators.maxLength, and Validators.pattern combinations can cover a lot of validation cases that can arise during the development of your Angular application. If the time comes where you can't handle your validation needs with the built-in validator, then you can build your very own validator.

In this section, we'll see how to validate that the movie_id field contains a valid entry (that is a number that is between one and four digits long) and that another movie does not already use the id. To do so, we can create the following class:

import { FormControl } from '@angular/forms';

interface ValidationResult {
 [key:string]:boolean;
}

export class MovieIDValidator{
    static idNotTaken(control: FormControl): ValidationResult { 

        let movies = require('./marvel-cinematic-
                universe...