Angular Reusable Custom Validator

Angular Reusable Custom Validator

In this topic we will discuss how to make a custom validator reusable in Angular.

The built-in validators in angular like the following, are reusable. This means we can use them with any form control on any angular form.  

• required
• min
• max
• minlength
• maxlength
• pattern

To be able to use one of the built-in angular validator, all we have to do is import the Validators class from '@angular/forms' package. 

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

Once the Validators class is imported, use the validator functions on the form control that you want to validate. 

fullName: ['', [Validators.required, Validators.minLength(2)]]

All the buil-in validator functions are marked as static functions in the Validators class. This allows us to use the validator functions, without the need to create an instance of the Validators class. 

Along the same lines let's make our emailDomain custom validator function reusable by including it as a static function in a separate class. 

We want to make this validator function available to all form controls on all forms. So, create a shared folder. In the shared folder, create a file with name custom.validators.ts and include the following code. 

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

export class CustomValidators {

    static emailDomain(domainName: string) {

        return (control: AbstractControl): { [key: string]: any } | null => {

            const email: string = control.value;

            const domain = email.substring(email.lastIndexOf('@') + 1);

            if (email === '' || domain.toLowerCase() === domainName.toLowerCase()) {

                return null;

            } else {

                return { 'emailDomain': true };

            }

        };

    }

}

Using the reusable Custom Validators : First, Import the CustomValidators class. Just like how we import the built-in Validators class, import the CustomValidators class from custom.validators.ts file. 

import { CustomValidators } from '../shared/custom.validators';

Tie the validator function to a form control that you want to validate. 

email: ['', [CustomValidators.emailDomain('dell.com')]]