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Random freezes, missing sound and display glitches usually trace back to one bad driver. Find and replace yours safely.Free scan · under a minuteFor a reactive Angular form, put a validator that compares sibling fields on their nearest common FormGroup. Read the child controls from the group and return either null when the combination is valid or a ValidationErrors object when it is not. The group then owns the error, so the template should check and display it on the group.
Reactive forms: validate related fields on their common group
A cross-field rule accepts or rejects a combination of values from different form fields—for example, when two fields must not match. Angular’s cross-field validation guide demonstrates this with sibling controls named name and role. Attach the validator to the FormGroup that contains the fields, rather than validating either field in isolation.
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Define a validator that returns a group error
A synchronous validator receives the control it is attached to. For a group-level validator, retrieve the child controls with get(), compare their values, and return an error map for an invalid combination or null for a valid one:
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import { AbstractControl, ValidationErrors, ValidatorFn } from '@angular/forms';
export const unambiguousRoleValidator: ValidatorFn =
(control: AbstractControl): ValidationErrors | null => {
const name = control.get('name');
const role = control.get('role');
if (!name || !role) {
return null;
}
return name.value === role.value
? { unambiguousRole: true }
: null;
};The !name || !role check makes the validator safe if it is reused with a group that does not contain both controls. Adapt the comparison and error key to the rule your form needs.
#1 Best Overall
Attach it when constructing the form
Pass the validator in the options for the group that contains the compared controls:
import { FormControl, FormGroup } from '@angular/forms';
actorForm = new FormGroup(
{
name: new FormControl(''),
role: new FormControl(''),
},
{ validators: unambiguousRoleValidator },
);Angular’s FormGroup API supports a validator function, an array of validators, or an options object containing validators and a validation trigger. If the fields are inside a nested group, attach the rule to the group that is their nearest common ancestor.
Display the error where it belongs
Because the validator is attached to actorForm, the error belongs to that group. Check the group with hasError() and, as in Angular’s documented example, wait until the group has been touched or changed before showing the message:
<div [formGroup]="actorForm">
<label>
Name
<input formControlName="name">
</label>
<label>
Role
<input formControlName="role">
</label>
<div
*ngIf="actorForm.hasError('unambiguousRole') && (actorForm.touched || actorForm.dirty)"
role="alert">
Name and role must be different.
</div>
</div>Showing the message beside the related fields or in a group-level error area makes the relationship clear. Avoid setting or removing errors on child controls inside the group validator as a shortcut: the documented pattern keeps the cross-field error on the group and renders it from there.
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Template-driven forms: register a validator directive
Template-driven forms use a different registration pattern. Angular’s form validation guide describes wrapping the validator function in a directive and providing that directive through NG_VALIDATORS. This lets Angular discover the validator through the template-driven form system; it is not the same as attaching a function in a reactive form’s model.
| Form approach | Where validation logic is declared | How it is registered |
|---|---|---|
| Reactive | In the component’s form model; a rule comparing siblings belongs on their common FormGroup. |
Pass the validator function or an array of functions in the group options. |
| Template-driven | In a directive that wraps the validator function. | Provide the directive through NG_VALIDATORS. |
When the rule needs asynchronous validation
Use an asynchronous validator when checking a combination requires an asynchronous result, such as a remote lookup. Angular’s validation guide states that asynchronous validators run only after synchronous validators pass, and each asynchronous validator must complete before Angular sets errors. Async validators return a Promise or Observable that emits an error map or null; synchronous validators return an error map or null.
Keep the error key and owner consistent
Choose a descriptive error key, such as unambiguousRole, and use that same key when checking the group in the template. With this ownership model, the validator reports whether the fields are invalid together, while each individual control can continue to report its own field-level errors.
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