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Angular has two types of validators:

## Synchronous validators

- Checked immediately, without waiting.
- Used most often.
- Examples of built-in ones:
  - `Validators.required` - the field must not be empty;
  - `Validators.email` - checks that the e-mail is correctly formatted;
  - `Validators.minLength(n)` / `Validators.maxLength(n)` - limit the length;
  - `Validators.pattern(regex)` - checks against a regular expression.
  - You can write your own, returning `null` (if all is fine) or an error object.

## Asynchronous validators

- Used when the check takes time, for example a request to the server.
- Return a `Promise` or `Observable`.
- Example: checking whether a username is already taken:

```typescript
  checkUsername(control: FormControl) {
    return this.api.checkUser(control.value).pipe(
      map(isTaken => (isTaken ? { usernameTaken: true } : null))
    );
  }
  ```

Both types can be applied to individual `FormControl` instances as well as to entire `FormGroup` instances.

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