Custom Validation in Angular Forms

Learn how to implement custom validators to ensure robust form validation in your Angular applications.

What is Custom Validation?

Custom validators let you define your own validation logic beyond built-in validators like required or minlength.

Creating a Custom Validator

A custom validator is a simple function that returns null if valid or an object if invalid:

function forbiddenNameValidator(control: AbstractControl): { [key: string]: any } | null {
  return control.value === 'admin' ? { 'forbiddenName': true } : null;
}

Applying the Validator

Apply the custom validator to your form control:

this.myForm = new FormGroup({
  username: new FormControl('', [forbiddenNameValidator])
});

Displaying Validation Errors

Show error messages conditionally in your template based on validation state:

<div *ngIf="myForm.get('username').hasError('forbiddenName')">
  Username 'admin' is not allowed.
</div>

Using Validator Directives

Validators can also be used directly as directives for template-driven forms:

<input name="username" ngModel forbiddenName>

Async Validators

Async validators let you perform validation involving asynchronous operations, such as server checks:

username: new FormControl('', null, [asyncValidator])

Testing Custom Validators

Always test validators to ensure they behave correctly under different scenarios and edge cases.

Summary

You've learned how to build and apply custom validators for robust form validation in Angular. Excellent work!