Get sibling AbstractControl from Angular reactive FormArray - Custom Validation

I'm having a Contact form in Angular using a Reactive form. The form contains firstName, lastName and an Array of address. Each address formgroup contains a checkbox, if the checkbox is checked, validation of the State text box mandatory is needed, along with min and max char length.

I have written a custom validator namely "stateValidator" and I tried to get the sibling element "isMandatory" but I am not able to get the control.

I tried the following approach

• control.root.get('isMandatory'); - Its returning null
• control.parent.get('isMandatory'); - Its throwing exception

I found some links in stackoverflow, but there is no answer available and some answers are not giving solutions, for example the code above: control.root.get('isMandatory'); I got this from one of the video tutorials but nothing worked.

The complete working source code is available in StackBlitz: https://stackblitz.com/edit/angular-custom-validators-in-dynamic-formarray

app.component.ts

import { Component } from '@angular/core';
import { FormBuilder, FormGroup, FormArray, Validators, AbstractControl } from '@angular/forms';

@Component({
  selector: 'my-app',
  templateUrl: './app.component.html',
  styleUrls: [ './app.component.css' ]
})
export class AppComponent  {
  name = 'Angular';
  public userForm: FormGroup;

  constructor(private _fb: FormBuilder) {
    this.userForm = this._fb.group({
      firstName: [],
      lastName: [],
      address: this._fb.array([this.addAddressGroup()])
    });
  }

  private addAddressGroup(): FormGroup {
    return this._fb.group({
      street: [],
      city: [],
      state: ['', this.stateValidator],
      isMandatory: [false, [Validators.required]]
    });
  }

  get addressArray(): FormArray {
    return <FormArray>this.userForm.get('address');
  }

  addAddress(): void {
    this.addressArray.push(this.addAddressGroup());
  }

  removeAddress(index: number): void {
    this.addressArray.removeAt(index);
  }

  stateValidator(control: AbstractControl): any {
        if(typeof control === 'undefined' || control === null 
        || typeof control.value === 'undefined' || control.value === null) {
            return {
                required: true
            }
        }

        const stateName: string = control.value.trim();
        const isPrimaryControl: AbstractControl = control.root.get('isMandatory');

        console.log(isPrimaryControl)

        if(typeof isPrimaryControl === 'undefined' || isPrimaryControl === null||
        typeof isPrimaryControl.value === 'undefined' || isPrimaryControl.value === null) {
            return {
                invalidFlag: true
            }
        }

        const isPrimary: boolean = isPrimaryControl.value;

        if(isPrimary === true) {
            if(stateName.length < 3) {
                return {
                    minLength: true
                };
            } else if(stateName.length > 50) {
                return {
                    maxLength: true
                };
            }
        } else {
            control.setValue('');
        }

        return null;
    }
}

app.component.html

<form class="example-form" [formGroup]="userForm">
  <div>
    <mat-card class="example-card">
      <mat-card-header>
        <mat-card-title>Users Creation</mat-card-title>
      </mat-card-header>
      <mat-card-content>
        <div class="primary-container">
          <mat-form-field>
            <input matInput placeholder="First Name" value="" formControlName="firstName">
          </mat-form-field>
          <mat-form-field>
            <input matInput placeholder="Last Name" value="" formControlName="lastName">
          </mat-form-field>
        </div>
        <div formArrayName="address">
          <div class="address-container" *ngFor="let group of addressArray.controls; let i = index;"
            [formGroupName]="i">
            <fieldset>
              <legend>
                <h3>Address: {{i + 1}}</h3>
              </legend>
              <mat-checkbox formControlName="isMandatory">Mandatory</mat-checkbox>
              <div>
                <mat-form-field>
                  <input matInput placeholder="Street" value="" formControlName="street">
                </mat-form-field>
                <mat-form-field>
                  <input matInput placeholder="City" value="" formControlName="city">
                </mat-form-field>
                <mat-form-field>
                  <input matInput placeholder="State" value="" formControlName="state">
                </mat-form-field>
              </div>
            </fieldset>
          </div>
        </div>
        <div class="form-row org-desc-parent-margin">
          <button mat-raised-button (click)="addAddress()">Add more address</button>
        </div>
      </mat-card-content>
    </mat-card>
  </div>
</form>
<mat-card class="pre-code">
  <mat-card-header>
    <mat-card-title>Users Information</mat-card-title>
  </mat-card-header>
  <mat-card-content>
    <pre>{{userForm.value | json}}</pre>
  </mat-card-content>
</mat-card>

Kindly assist me how to get the sibling abstract control in the custom validator method.

I tried the code which was specified in the following couple of answers

isPrimaryControl = (<FormGroup>control.parent).get('isMandatory')

It's throwing an error ERROR Error: too much recursion. Kindly assist me how to fix this.

You may need to cast the parent to FormGroup, try using :

if(control.parent) {
    (<FormGroup>control.parent).get('isMandatory')
}