requiredIf

The field is required when a sibling field is set, optionally only when the sibling matches a specific value.

Signature

NguardValidators.CrossField.requiredIf(
    fieldKey: string,
    value?: primitive,
    isStrict?: boolean,
): ValidatorFn

Parameter	Type	Default	Description
fieldKey	string	—	The key of the sibling field whose value triggers the requirement
value	primitive (optional)	undefined	If provided, the sibling must equal this value to trigger the requirement
isStrict	boolean	false	If true, the equality check against value is strict (===)

Reactive forms

import { FormControl, FormGroup } from '@angular/forms';
import { NguardValidators } from 'ng-nguard';

// Required whenever 'paymentMethod' is set to anything truthy
new FormGroup({
    paymentMethod: new FormControl(''),
    cardNumber: new FormControl('', [
        NguardValidators.CrossField.requiredIf('paymentMethod'),
    ]),
});

// Required only when 'paymentMethod' is exactly 'creditCard'
new FormGroup({
    paymentMethod: new FormControl(''),
    cardNumber: new FormControl('', [
        NguardValidators.CrossField.requiredIf('paymentMethod', 'creditCard'),
    ]),
});

// Strict equality against a numeric trigger
new FormGroup({
    accountType: new FormControl(''),
    companyName: new FormControl('', [
        NguardValidators.CrossField.requiredIf('accountType', 1, true),
    ]),
});

Template-driven forms

<!-- Shorthand: required if 'paymentMethod' is truthy -->
<input ngModel name="cardNumber" [nguardRequiredIf]="'paymentMethod'" />

<!-- Object form: required if sibling equals a specific value -->
<input
    ngModel
    name="cardNumber"
    [nguardRequiredIf]="{ fieldKey: 'paymentMethod', value: 'creditCard' }"
/>

<!-- Strict equality against a numeric value -->
<input
    ngModel
    name="companyName"
    [nguardRequiredIf]="{ fieldKey: 'accountType', value: 1, isStrict: true }"
/>

Error key

{ requiredIf: true }

Notes

• "Required" here means the field's value must be truthy after type coercion (non-empty string, non-zero number, non-null/undefined).
• When the trigger value is omitted, any truthy sibling value triggers the requirement.
• When the trigger value is provided and isStrict: false (default), == is used — '1' matches 1.
• When the sibling has no value (or its parent form group doesn't exist), the requirement is not triggered, so the field can be empty.

See also

• v0.5.0 conditional siblings: requiredUnless, requiredWith, requiredWithout, prohibitedIf (planned)