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.

In the ngFor statement, you are defining the variable "feature" to use for each control of the form array.

*ngFor="let feature of features.controls; let index = index; let last = last"

You can use that variable to get the invalid status of that control.

feature.invalid

Here is the Stackblitz

Additionally

You don't need the required attribute in the HTML when you are using reactive forms.

So this

<input type="text" class="form-control px-2 col" [formControlName]="index" title="feature" required>

Can be

<input type="text" class="form-control px-2 col" [formControlName]="index" title="feature">

The FormGroup class has a get method which returns the abstractControl for the given key. The one you use in formControlName.

Here the links for both Api docs:
AbstractControl
FormGroup

<form [formGroup]="form">
  <input formControlName="name" type="text" />
  <div *ngIf="form.get('name').invalid">
    <p><Message you like to show/p>
  </div>
</form>

You Should use form controls to achieve that sharply.

<div *ngIf="formGroup.get('features').controls[i].controls.index.invalid && (formGroup.get('features').controls[i].controls.index.dirty || formGroup.get('features').controls[i].controls.index.touched)"                                          class="text-center error">
<p [hidden]="!formGroup.get('features').controls[i].controls.index.errors.required">Index is required.</p>
</div>

As above answer using