Here is the way to notify user that there are fields with non-valid values.
markFieldsAsTouched function FormGroup or FormArray as an argument.
function markFieldsAsTouched(form: AbstractControl): void {
form.markAsTouched({ onlySelf: true });
if (form instanceof FormArray || form instanceof FormGroup) {
Object.values(form.controls).forEach(markFieldsAsTouched);
}
}
It's very useful to check out more general method Accessing all nested form controls by Thekiba to work with controls.