*`#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/forms/AbstractControl)
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.
* **Sync validators**: 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`.
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/forms/Validators) API reference.
* The name control sets up two built-in validators—`Validators.required` and `Validators.minLength(4)`—and one custom validator, `forbiddenNameValidator`. For more details see the [Custom validators](guide/form-validation#custom-validators) section in this guide.
* This example 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 shorthands
<code-examplepath="form-validation/src/app/reactive/hero-form-reactive.component.html"region="name-with-error-msg"title="reactive/hero-form-reactive.component.html (name with error msg)"linenums="false">
Once the `ForbiddenValidatorDirective` is ready, you can simply add its selector, `appForbiddenName`, to any input element to activate it. For example:
Like in AngularJS, 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:
In the following section, we will make sure that our heroes do not reveal their true identities by filling out the Hero Form. We will do that by validating that the hero names and alter egos do not match.
Notice that the name and alterEgo are sibling controls. To evaluate both controls in a single custom validator, we should perform the validation in a common ancestor control: the `FormGroup`. That way, we can query the `FormGroup` for the child controls which will allow us to compare their values.
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.
First we retrieve the child controls by calling the `FormGroup`'s [get](api/forms/AbstractControl#get) method. Then we simply compare the values of the `name` and `alterEgo` controls.
If the values do not match, the hero's identity remains secret, and we can safely return null. Otherwise, the hero's identity is revealed and we must mark the form as invalid by returning an error object.
First we must create a directive that will wrap the validator function. We provide it as the validator using the `NG_VALIDATORS` token. If you are not sure why, or you do not fully understand the syntax, revisit the previous [section](guide/form-validation#adding-to-template-driven-forms).
Next, we have to add the directive to the html template. Since the validator must be registered at the highest level in the form, we put the directive on the `form` tag.
This section shows how to create asynchronous validators. It assumes some basic knowledge of creating [custom validators](guide/form-validation#custom-validators).
### The Basics
Just like synchronous validators have the `ValidatorFn` and `Validator` interfaces, asynchronous validators have their own counterparts: `AsyncValidatorFn` and `AsyncValidator`.
They are very similar with the only difference being:
* They 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`.
It is important to note that the 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 more basic validation methods fail.
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.
A common UI pattern is to show a spinner while the async validation is being performed. The following example presents how to achieve this with template-driven forms:
In the following section, validation is performed asynchronously to ensure that our heroes pick an alter ego that is not already taken. New heroes are constantly enlisting and old heroes are leaving the service. That means that we do not have the list of available alter egos ahead of time.
To validate the potential alter ego, we need to consult a central database of all currently enlisted heroes. The process is asynchronous, so we need a special validator for that.
As you can see, the `UniqueAlterEgoValidator` class implements the `AsyncValidator` interface. In the constructor, we inject the `HeroesService` that has the following interface:
In a real world application, the `HeroesService` is 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 we 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. We pipe the response through the `map` operator and transform it into a validation result. As always, we return `null` if the form is valid, and `ValidationErrors` if it is not. We make sure to handle any potential errors with the `catchError` operator.
Here we decided that `isAlterEgoTaken()` error is treated 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 async validation is done. The `pending` flag is set to `false`, and the form validity is updated.
### Note on performance
By default, all validators are run after every form value change. With synchronous validators, this will not likely have a noticeable impact on application performance. However, it's common for async validators to 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.
We can delay updating the form validity by changing the `updateOn` property from `change` (default) to `submit` or `blur`.
