Side effects for event triggers | TanStack Form Angular Docs

For situations where you want to "affect" or "react" to triggers, there's the listener API. For example, if you, as the developer, want to reset a form field as a result of another field changing, you would use the listener API.

Imagine the following user flow:

User selects a country from a drop-down.
User then selects a province from another drop-down.
User changes the selected country to a different one.

In this example, when the user changes the country, the selected province needs to be reset as it's no longer valid. With the listener API, we can subscribe to the onChange event and dispatch a reset to the field "province" when the listener is fired.

Events that can be "listened" to are:

onChange
onBlur
onMount
onSubmit
onUnmount

angular-ts

@Component({
  selector: 'app-root',
  standalone: true,
  imports: [TanStackField],
  template: `
    <ng-container
      [tanstackField]="form"
      name="country"
      [listeners]="{
        onChange: onCountryChange,
      }"
      #country="field"
    ></ng-container>

    <ng-container
      [tanstackField]="form"
      name="province"
      #province="field"
    ></ng-container>
  `,
})
export class AppComponent {
  form = injectForm({
    defaultValues: {
      country: '',
      province: '',
    },
  })

  onCountryChange: FieldListenerFn<any, any, any, any, string> = ({
    value,
  }) => {
    console.log(`Country changed to: ${value}, resetting province`)
    this.form.setFieldValue('province', '')
  }
}

@Component({
  selector: 'app-root',
  standalone: true,
  imports: [TanStackField],
  template: `
    <ng-container
      [tanstackField]="form"
      name="country"
      [listeners]="{
        onChange: onCountryChange,
      }"
      #country="field"
    ></ng-container>

    <ng-container
      [tanstackField]="form"
      name="province"
      #province="field"
    ></ng-container>
  `,
})
export class AppComponent {
  form = injectForm({
    defaultValues: {
      country: '',
      province: '',
    },
  })

  onCountryChange: FieldListenerFn<any, any, any, any, string> = ({
    value,
  }) => {
    console.log(`Country changed to: ${value}, resetting province`)
    this.form.setFieldValue('province', '')
  }
}