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Validating form input

You can improve overall data quality by validating user input for accuracy and completeness. This page shows how to validate user input from the UI and display useful validation messages, in both reactive and template-driven forms.

Prerequisites

Before reading about form validation, you should have a basic understanding of the following.

Get the complete example code for the reactive and template-driven forms used here to illustrate form validation. Run the live example.

Validating input in template-driven forms

To add validation to a template-driven form, you add the same validation attributes as you would with native HTML form validation. Angular uses directives to match these attributes with validator functions in the framework.

Every time the value of a form control changes, Angular runs validation and generates either a list of validation errors that results in an INVALID status, or null, which results in a VALID status.

You can then inspect the control's state by exporting ngModel to a local template variable. The following example exports NgModel into a variable called name:

<input id="name" name="name" class="form-control"
      required minlength="4" appForbiddenName="bob"
      [(ngModel)]="hero.name" #name="ngModel" >

<div *ngIf="name.invalid && (name.dirty || name.touched)"
    class="alert alert-danger">

  <div *ngIf="name.errors.required">
    Name is required.
  </div>
  <div *ngIf="name.errors.minlength">
    Name must be at least 4 characters long.
  </div>
  <div *ngIf="name.errors.forbiddenName">
    Name cannot be Bob.
  </div>

</div>

Notice the following features illustrated by the example.

  • The <input> element carries the HTML validation attributes: required and minlength. It also carries a custom validator directive, forbiddenName. For more information, see the Custom validators section.

  • #name="ngModel" exports NgModel into a local variable called name. NgModel mirrors many of the properties of its underlying FormControl instance, so you can use this in the template to check for control states such as valid and dirty. For a full list of control properties, see the AbstractControl API reference.

    • The *ngIf on the <div> element reveals a set of nested message divs but only if the name is invalid and the control is either dirty or touched.

    • Each nested <div> can present a custom message for one of the possible validation errors. There are messages for required, minlength, and forbiddenName.

To prevent the validator from displaying errors before the user has a chance to edit the form, you should check for either the dirty or touched states in a control.

  • When the user changes the value in the watched field, the control is marked as "dirty".
  • When the user blurs the form control element, the control is marked as "touched".

Validating input in reactive forms

In a reactive form, the source of truth is the component class. Instead of adding validators through attributes in the template, you add validator functions directly to the form control model in the component class. Angular then calls these functions whenever the value of the control changes.

Validator functions

Validator functions can be either synchronous or asynchronous.

  • Sync validators: Synchronous functions that take a control instance and immediately return either a set of validation errors or null. You can pass these in as the second argument when you instantiate a FormControl.

  • Async validators: Asynchronous functions that take a control instance and return a Promise or Observable that later emits a set of validation errors or null. You can pass these in as the third argument when you instantiate a FormControl.

For performance reasons, Angular only runs async validators if all sync validators pass. Each must complete before errors are set.

Built-in validator functions

You can choose to write your own validator functions, or you can use some of Angular's built-in validators.

The same built-in validators that are available as attributes in template-driven forms, such as required and minlength, are all available to use as functions from the Validators class. For a full list of built-in validators, see the Validators API reference.

To update the hero form to be a reactive form, you can use some of the same built-in validators—this time, in function form, as in the following example.

ngOnInit(): void {
  this.heroForm = new FormGroup({
    name: new FormControl(this.hero.name, [
      Validators.required,
      Validators.minLength(4),
      forbiddenNameValidator(/bob/i) // <-- Here's how you pass in the custom validator.
    ]),
    alterEgo: new FormControl(this.hero.alterEgo),
    power: new FormControl(this.hero.power, Validators.required)
  });

}

get name() { return this.heroForm.get('name'); }

get power() { return this.heroForm.get('power'); }

In this example, the name control sets up two built-in validators—Validators.required and Validators.minLength(4)—and one custom validator, forbiddenNameValidator. (For more details see custom validators below.)

All of these validators are synchronous, so they are passed as the second argument. Notice that you can support multiple validators by passing the functions in as an array.

This example also adds a few getter methods. In a reactive form, you can always access any form control through the get method on its parent group, but sometimes it's useful to define getters as shorthand for the template.

If you look at the template for the name input again, it is fairly similar to the template-driven example.

<input id="name" class="form-control"
      formControlName="name" required >

<div *ngIf="name.invalid && (name.dirty || name.touched)"
    class="alert alert-danger">

  <div *ngIf="name.errors.required">
    Name is required.
  </div>
  <div *ngIf="name.errors.minlength">
    Name must be at least 4 characters long.
  </div>
  <div *ngIf="name.errors.forbiddenName">
    Name cannot be Bob.
  </div>
</div>

This form differs from the template-driven version in that it no longer exports any directives. Instead, it uses the name getter defined in the component class.

Notice that the required attribute is still present in the template. Although it's not necessary for validation, it should be retained to for accessibility purposes.

Defining custom validators

The built-in validators don't always match the exact use case of your application, so you sometimes need to create a custom validator.

Consider the forbiddenNameValidator function from previous reactive-form examples. Here's what the definition of that function looks like.

/** A hero's name can't match the given regular expression */
export function forbiddenNameValidator(nameRe: RegExp): ValidatorFn {
  return (control: AbstractControl): {[key: string]: any} | null => {
    const forbidden = nameRe.test(control.value);
    return forbidden ? {forbiddenName: {value: control.value}} : null;
  };
}

The function is a factory that takes a regular expression to detect a specific forbidden name and returns a validator function.

In this sample, the forbidden name is "bob", so the validator will reject any hero name containing "bob". Elsewhere it could reject "alice" or any name that the configuring regular expression matches.

The forbiddenNameValidator factory returns the configured validator function. That function takes an Angular control object and returns either null if the control value is valid or a validation error object. The validation error object typically has a property whose name is the validation key, 'forbiddenName', and whose value is an arbitrary dictionary of values that you could insert into an error message, {name}.

Custom async validators are similar to sync validators, but they must instead return a Promise or observable that later emits null or a validation error object. In the case of an observable, the observable must complete, at which point the form uses the last value emitted for validation.

Adding custom validators to reactive forms

In reactive forms, add a custom validator by passing the function directly to the FormControl.

this.heroForm = new FormGroup({
  name: new FormControl(this.hero.name, [
    Validators.required,
    Validators.minLength(4),
    forbiddenNameValidator(/bob/i) // <-- Here's how you pass in the custom validator.
  ]),
  alterEgo: new FormControl(this.hero.alterEgo),
  power: new FormControl(this.hero.power, Validators.required)
});

Adding custom validators to template-driven forms

In template-driven forms, add a directive to the template, where the directive wraps the validator function. For example, the corresponding ForbiddenValidatorDirective serves as a wrapper around the forbiddenNameValidator.

Angular recognizes the directive's role in the validation process because the directive registers itself with the NG_VALIDATORS provider, as shown in the following example. NG_VALIDATORS is a predefined provider with an extensible collection of validators.

providers: [{provide: NG_VALIDATORS, useExisting: ForbiddenValidatorDirective, multi: true}]

The directive class then implements the Validator interface, so that it can easily integrate with Angular forms. Here is the rest of the directive to help you get an idea of how it all comes together.

@Directive({
  selector: '[appForbiddenName]',
  providers: [{provide: NG_VALIDATORS, useExisting: ForbiddenValidatorDirective, multi: true}]
})
export class ForbiddenValidatorDirective implements Validator {
  @Input('appForbiddenName') forbiddenName: string;

  validate(control: AbstractControl): {[key: string]: any} | null {
    return this.forbiddenName ? forbiddenNameValidator(new RegExp(this.forbiddenName, 'i'))(control)
                              : null;
  }
}

Once the ForbiddenValidatorDirective is ready, you can add its selector, appForbiddenName, to any input element to activate it. For example:

<input id="name" name="name" class="form-control"
      required minlength="4" appForbiddenName="bob"
      [(ngModel)]="hero.name" #name="ngModel" >

Notice that the custom validation directive is instantiated with useExisting rather than useClass. The registered validator must be this instance of the ForbiddenValidatorDirective—the instance in the form with its forbiddenName property bound to “bob".

If you were to replace useExisting with useClass, then you’d be registering a new class instance, one that doesn’t have a forbiddenName.

Control status CSS classes

Angular automatically mirrors many control properties onto the form control element as CSS classes. You can use these classes to style form control elements according to the state of the form. The following classes are currently supported.

  • .ng-valid
  • .ng-invalid
  • .ng-pending
  • .ng-pristine
  • .ng-dirty
  • .ng-untouched
  • .ng-touched

In the following example, the hero form uses the .ng-valid and .ng-invalid classes to set the color of each form control's border.

.ng-valid[required], .ng-valid.required  {
  border-left: 5px solid #42A948; /* green */
}

.ng-invalid:not(form)  {
  border-left: 5px solid #a94442; /* red */
}

Cross-field validation

A cross-field validator is a custom validator that compares the values of different fields in a form and accepts or rejects them in combination. For example, you might have a form that offers mutually incompatible options, so that if the user can choose A or B, but not both. Some field values might also depend on others; a user might be allowed to choose B only if A is also chosen.

The following cross validation examples show how to do the following:

  • Validate reactive or template-based form input based on the values of two sibling controls,
  • Show a descriptive error message after the user interacted with the form and the validation failed.

The examples use cross-validation to ensure that heroes do not reveal their true identities by filling out the Hero Form. The validators do this by checking that the hero names and alter egos do not match.

Adding cross-validation to reactive forms

The form has the following structure:

const heroForm = new FormGroup({
  'name': new FormControl(),
  'alterEgo': new FormControl(),
  'power': new FormControl()
});

Notice that the name and alterEgo are sibling controls. To evaluate both controls in a single custom validator, you must perform the validation in a common ancestor control: the FormGroup. You query the FormGroup for its child controls so that you can compare their values.

To add a validator to the FormGroup, pass the new validator in as the second argument on creation.

const heroForm = new FormGroup({
  'name': new FormControl(),
  'alterEgo': new FormControl(),
  'power': new FormControl()
}, { validators: identityRevealedValidator });

The validator code is as follows.

/** A hero's name can't match the hero's alter ego */
export const identityRevealedValidator: ValidatorFn = (control: FormGroup): ValidationErrors | null => {
  const name = control.get('name');
  const alterEgo = control.get('alterEgo');

  return name && alterEgo && name.value === alterEgo.value ? { identityRevealed: true } : null;
};

The identity validator implements the ValidatorFn interface. It takes an Angular control object as an argument and returns either null if the form is valid, or ValidationErrors otherwise.

The validator retrieves the child controls by calling the FormGroup's get method, then compares the values of the name and alterEgo controls.

If the values do not match, the hero's identity remains secret, both are valid, and the validator returns null. If they do match, the hero's identity is revealed and the validator must mark the form as invalid by returning an error object.

To provide better user experience, the template shows an appropriate error message when the form is invalid.

<div *ngIf="heroForm.errors?.identityRevealed && (heroForm.touched || heroForm.dirty)" class="cross-validation-error-message alert alert-danger">
    Name cannot match alter ego.
</div>

This *ngIf displays the error if the FormGroup has the cross validation error returned by the identityRevealed validator, but only if the user has finished interacting with the form.

Adding cross-validation to template-driven forms

For a template-driven form, you must create a directive to wrap the validator function. You provide that directive as the validator using the NG_VALIDATORS token, as shown in the following example.

@Directive({
  selector: '[appIdentityRevealed]',
  providers: [{ provide: NG_VALIDATORS, useExisting: IdentityRevealedValidatorDirective, multi: true }]
})
export class IdentityRevealedValidatorDirective implements Validator {
  validate(control: AbstractControl): ValidationErrors {
    return identityRevealedValidator(control);
  }
}

You must add the new directive to the HTML template. Because the validator must be registered at the highest level in the form, the following template puts the directive on the form tag.

<form #heroForm="ngForm" appIdentityRevealed>

To provide better user experience, we show an appropriate error message when the form is invalid.

<div *ngIf="heroForm.errors?.identityRevealed && (heroForm.touched || heroForm.dirty)" class="cross-validation-error-message alert alert-danger">
    Name cannot match alter ego.
</div>

This is the same in both template-driven and reactive forms.

Creating asynchronous validators

Asynchronous validators implement the AsyncValidatorFn and AsyncValidator interfaces. These are very similar to their synchronous counterparts, with the following differences.

  • The validate() functions must return a Promise or an observable,
  • The observable returned must be finite, meaning it must complete at some point. To convert an infinite observable into a finite one, pipe the observable through a filtering operator such as first, last, take, or takeUntil.

Asynchronous validation happens after the synchronous validation, and is performed only if the synchronous validation is successful. This check allows forms to avoid potentially expensive async validation processes (such as an HTTP request) if the more basic validation methods have already found invalid input.

After asynchronous validation begins, the form control enters a pending state. You can inspect the control's pending property and use it to give visual feedback about the ongoing validation operation.

A common UI pattern is to show a spinner while the async validation is being performed. The following example shows how to achieve this in a template-driven form.

<input [(ngModel)]="name" #model="ngModel" appSomeAsyncValidator>
<app-spinner *ngIf="model.pending"></app-spinner>

Implementing a custom async validator

In the following example, an async validator ensures that heroes pick an alter ego that is not already taken. New heroes are constantly enlisting and old heroes are leaving the service, so the list of available alter egos cannot be retrieved ahead of time. To validate the potential alter ego entry, the validator must initiate an asynchronous operation to consult a central database of all currently enlisted heroes.

The following code create the validator class, UniqueAlterEgoValidator, which implements the AsyncValidator interface.

@Injectable({ providedIn: 'root' })
export class UniqueAlterEgoValidator implements AsyncValidator {
  constructor(private heroesService: HeroesService) {}

  validate(
    ctrl: AbstractControl
  ): Promise<ValidationErrors | null> | Observable<ValidationErrors | null> {
    return this.heroesService.isAlterEgoTaken(ctrl.value).pipe(
      map(isTaken => (isTaken ? { uniqueAlterEgo: true } : null)),
      catchError(() => of(null))
    );
  }
}

The constructor injects the HeroesService, which defines the following interface.

interface HeroesService {
  isAlterEgoTaken: (alterEgo: string) => Observable<boolean>;
}

In a real world application, the HeroesService would be responsible for making an HTTP request to the hero database to check if the alter ego is available. From the validator's point of view, the actual implementation of the service is not important, so the example can just code against the HeroesService interface.

As the validation begins, the UniqueAlterEgoValidator delegates to the HeroesService isAlterEgoTaken() method with the current control value. At this point the control is marked as pending and remains in this state until the observable chain returned from the validate() method completes.

The isAlterEgoTaken() method dispatches an HTTP request that checks if the alter ego is available, and returns Observable<boolean> as the result. The validate() method pipes the response through the map operator and transforms it into a validation result.

The method then, like any validator, returns null if the form is valid, and ValidationErrors if it is not. This validator handles any potential errors with the catchError operator. In this case, the validator treats the isAlterEgoTaken() error as a successful validation, because failure to make a validation request does not necessarily mean that the alter ego is invalid. You could handle the error differently and return the ValidationError object instead.

After some time passes, the observable chain completes and the asynchronous validation is done. The pending flag is set to false, and the form validity is updated.

Optimizing performance of async validators

By default, all validators run after every form value change. With synchronous validators, this does not normally have a noticeable impact on application performance. Async validators, however, commonly perform some kind of HTTP request to validate the control. Dispatching an HTTP request after every keystroke could put a strain on the backend API, and should be avoided if possible.

You can delay updating the form validity by changing the updateOn property from change (default) to submit or blur.

With template-driven forms, set the property in the template.

<input [(ngModel)]="name" [ngModelOptions]="{updateOn: 'blur'}">

With reactive forms, set the property in the FormControl instance.

new FormControl('', {updateOn: 'blur'});

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Licensed under the Creative Commons Attribution License 4.0.
https://angular.io/guide/form-validation