Angular Form Array: Complete Guide - blog.angular-university.io/angular-form-array/

01.ts

form = this.fb.group({
    title: ['', {
        validators: [
            Validators.required,
            Validators.minLength(5),
            Validators.maxLength(60)
        ],
        asyncValidators: [courseTitleValidator(this.courses)],
        updateOn: 'blur'
    }],
    releasedAt: [new Date(), Validators.required],
    category: ['BEGINNER', Validators.required],
    downloadsAllowed: [false, Validators.requiredTrue],
    longDescription: ['', [Validators.required, Validators.minLength(3)]]
});

02.ts

@Component({
  selector: 'form-array-example',
  templateUrl: 'form-array-example.component.html',
  styleUrls: ['form-array-example.component.scss']
})
export class FormArrayExampleComponent {

    form = this.fb.group({
        ... other form controls ...
        lessons: this.fb.array([])
    });

    constructor(private fb:FormBuilder) {}
  
    get lessons() {
      return this.form.controls["lessons"] as FormArray;
    }
}

03.html

<button mat-mini-fab (click)="addLesson()">
  <mat-icon class="add-course-btn">add</mat-icon>
</button>

04.ts

addLesson() {
  const lessonForm = this.fb.group({
      title: ['', Validators.required],
      level: ['beginner', Validators.required]
  });

  this.lessons.push(lessonForm);
}

05.ts

deleteLesson(lessonIndex: number) {
    this.lessons.removeAt(lessonIndex);
}

06.html

<h3>Add Course Lessons:</h3>
<div class="add-lessons-form" [formGroup]="form">
    <ng-container formArrayName="lessons">
        <ng-container *ngFor="let lessonForm of lessons.controls; let i = index">
            <div class="lesson-form-row" [formGroup]="lessonForm">
                <mat-form-field appearance="fill">
                    <input matInput
                           formControlName="title"
                           placeholder="Lesson title">
                </mat-form-field>
                <mat-form-field appearance="fill">
                    <mat-select formControlName="level"
                            placeholder="Lesson level">
                        <mat-option value="beginner">Beginner</mat-option>
                        <mat-option value="intermediate">Intermediate</mat-option>
                        <mat-option value="advanced">Advanced</mat-option>
                    </mat-select>
                </mat-form-field>
                <mat-icon class="delete-btn" (click)="deleteLesson(i)">
                    delete_forever</mat-icon>
            </div>
        </ng-container>
    </ng-container>
  
    <button mat-mini-fab (click)="addLesson()">
        <mat-icon class="add-course-btn">add</mat-icon>
    </button>

</div>

07.ts

@Component({
  selector: 'form-array-example',
  templateUrl: 'form-array-example.component.html',
  styleUrls: ['form-array-example.component.scss']
})
export class FormArrayExampleComponent {

    form = this.fb.group({
        ... other form controls ...
        lessons: this.fb.array([])
    });

    constructor(private fb:FormBuilder) {}
  
    get lessons() {
      return this.form.controls["lessons"] as FormArray;
    }

    addLesson() {
      const lessonForm = this.fb.group({
        title: ['', Validators.required],
        level: ['beginner', Validators.required]
      });
      this.lessons.push(lessonForm);
    }

    deleteLesson(lessonIndex: number) {
      this.lessons.removeAt(lessonIndex);
    }
}

For me the error disappears by casting the lessonsForm to FormGroup. To do this I created a method to do the casting and used it in the template.

In the template: <div class="lesson-form-row" [formGroup]="toFormGroup(lessonForm)">

In the class: toFormGroup = (form: AbstractControl) => form as FormGroup;

I am wondering the same

The error Type 'AbstractControl<any, any>' is missing the following properties from type 'FormGroup<any>' happens when you have in tsconfig.json:

  "angularCompilerOptions": {
    "strictTemplates": true
  },

Solution with pipe (I couldn't manage to make it work with generic pipe like in https://stackoverflow.com/a/69466380/588759):

import { Pipe, PipeTransform } from '@angular/core';
import { AbstractControl, FormGroup } from '@angular/forms';

/**
 * https://stackoverflow.com/a/72517767/588759
 * https://stackoverflow.com/a/67059770/588759
 */

@Pipe({
  name: 'formGroup',
})
export class FormGroupPipe implements PipeTransform {
  transform(value: AbstractControl): FormGroup<(typeof value)['value']> {
    return value as FormGroup<(typeof value)['value']>;
  }
}

and use as [formGroup]="lessonForm | formGroup"