I have Angular form that is built with help of FormBuilder.

Form contains a FormArray which has as many fields as user wants. I've set validator for fields

this.fb.array([this.fb.control('', Validators.required)])

and for each new push validator is the same.

The problem is that I don't know how to access a specific field's isValid property since they are bound with FormControl via [formControlName]="index".

I've tried to do it that way, but it doesn't seem to work

<div *ngIf="array.at(index).invalid" class="alert alert-danger p-2">
</div>

Where array is a formArray.controls passed from a parent.

Update #1

There is a case https://stackblitz.com/edit/angular-7ppkoh

I don't really think this would be possible completely on the template. That's because to access the FormArray's control's state in your template, you'll have to access this.formGroup.get('features').controls[i].invalid. But since get returns an instance of type AbstractControl, you won't have access to the controls array on it. For that, you'll have to typecast whatever is returned from this.formGroup.get('features') into a FormArray. And I don't really think that would be possible on the template.

You'll have to create a method in your class that would return the validity of the control based on the index.

So if we continue to refer to your stackblitz eg, here's how:

<form [formGroup]="formGroup">
  <div formArrayName="features">
    <div 
      class="row no-gutters form-group" 
      *ngFor="let feature of features.controls; let index = index; let last = last">
      <input 
        type="text" 
        class="form-control px-2 col" 
        [formControlName]="index" 
        title="feature" 
        required>

        IS Invalid - {{ getValidity(index) }}

      <div class="col-3 col-md-2 row no-gutters">
        <button 
          class="col btn btn-outline-danger" 
          (click)="removeFeature(index)">
          -
        </button>
        <button 
          class="col btn btn-success" 
          *ngIf="last" 
          (click)="addFeature()">
          +
        </button>
      </div>
    </div>
  </div>
</form>

And in your class:

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

@Component({
  selector: 'my-app',
  templateUrl: './app.component.html',
  styleUrls: [ './app.component.css' ]
})
export class AppComponent  {

  constructor(
    private fb: FormBuilder,
  ) {}

  formGroup = this.fb.group({
    features: this.fb.array([this.fb.control('', Validators.required)])
  });

  get features(): FormArray {
    return this.formGroup.get('features') as FormArray;
  }

  addFeature(): void {
    this.features.push(this.fb.control('', Validators.required));
  }

  getValidity(i) {
    return (<FormArray>this.formGroup.get('features')).controls[i].invalid;
  }

  removeFeature(index): void {
    this.features.removeAt(index);
    console.log(this.features);
  }

}

UPDATE

A few months back I realized that calling a method in one of the data-binding syntaxes(i.e. String Interpolation - {{ ... }}, Property Binding - [propertyName]="methodName()", or Attribute Binding - [class.class-name]="methodName()" OR [style.styleName]="methodName()") is extremely costly as far as performance is concerned.

So, you should do it using:

{{ formGroup.controls['features'].controls[index].invalid }}

Instead of:

{{ getValidity(index) }}

Here's an Updated Working Sample StackBlitz for your ref.

If you wanna know more about it, I highly recommend you to check this thread out:

Angular Performance: DOM Event causes unnecessary function calls

Hope this helps :)