Angular2: reactive, nested form

nested-form.component.ts

import { Component, EventEmitter, Input, OnInit, Output } from '@angular/core'
import { AbstractControl, FormArray, FormBuilder, FormControl,
  FormGroup, Validators } from '@angular/forms'

@Component({
  selector: 'nested-form',
  template:
`
<form [formGroup]="myForm" (ngSubmit)="submit()">
  <h4>Form</h4>
  <hr>
  <fieldset formArrayName="items">
    <h6>Items</h6>
    <div *ngIf="myForm.get('items').hasError('minSum')">
      You must buy a total sum of at least {{ myForm.get('items').getError('minSum') }}.
    </div>
    <div class="form-group row"
         *ngFor="let item of myForm.get('items').controls; let i=index"
         [formGroup]="item">
      <div class="col-sm-6">
        <label [attr.for]="'name'+i">Name</label>
        <input type="text" class="form-control"
               [attr.id]="'name'+i" formControlName="name">
      </div>
      <div class="col-sm-5">
        <label [attr.for]="'quantity'+i">Quantity</label>
        <input type="number" class="form-control"
               [attr.id]="'quantity'+i" formControlName="quantity">
      </div>
      <div class="col-sm-1 py-1">
        <button type="button" class="btn"
                (click)="myForm.get('items').removeAt(i)">-</button>
      </div>
    </div>
  </fieldset>
  <div class="form-group row">
    <div class="col-sm-2 col-sm-offset-10">
      <button type="button" class="btn btn-link"
              (click)="myForm.get('items').push(buildItem(''))">Add another item</button>
    </div>
  </div>
  <hr>
  <div class="form-group">
    <input type="submit" class="form-control"
           value="Submit" [disabled]="myForm?.invalid">
  </div>
</form>
<h6 class="mt-3">FormGroup Model (<code>myForm.value</code>)</h6>
<div><pre><code>{{ myForm?.value | json }}</code></pre></div>
`
})
export class NestedFormComponent implements OnInit {

  myForm: FormGroup;

  constructor(
    private fb: FormBuilder
  ) {}

  ngOnInit() {
    // build the form model
    this.myForm = this.fb.group({
      items: this.fb.array(
        [this.buildItem('aaa'), this.buildItem('')],
        ItemsValidator.minQuantitySum(300)
    })
  }

  submit() {
    console.log("Reactive Form submitted: ", this.myForm)
  }

  buildItem(val: string) {
    return new FormGroup({
      name: new FormControl(val, Validators.required),
      quantity: new FormControl(100)
    })
  }
}

class ItemsValidator {

  static minQuantitySum(val: number) {
    return (c: AbstractControl) => {
      let sum = c.value
        .map(item => item.quantity)
        .reduce((acc, cur) => acc + cur, 0 );
      if (sum < val) {
        return { minSum: val }
      }
    })
  }
}