title: Sum of my ngModel elements returns NAN when formGroup.length > 1
body: <p>Outside my form-group i have this ngModel input:</p>
  
  <pre><code>&lt;input [class.disabled]="controls[0] &gt; 0" [(ngModel)]="produto.estoque" type="text"&gt;
  </code></pre>
  
  <p>This input show the sum of the input values inside my formgroup.</p>
  
  <p>I have this form group:</p>
  
  <pre><code>&lt;form [formGroup]="variacaoForm"&gt;
    &lt;ng-container *ngFor="let item of variacoes.controls; let i = index;"&gt;
      &lt;input [(ngModel)]="controls[i]" (keyup)="soma($event)" type="text" name="estoque_variacao{{i}}" formControlName="estoque_variacao"&gt;
  </code></pre>
  
  <p>I can have "N" inputs.</p>
  
  <p>This is actualy my function that sum the input estoque_variacao:</p>
  
  <pre><code>  soma(){
      let sum = 0;
  
      Object.keys(this.variacaoForm.value.variacoes).map(key =&gt; {
        sum +=  +this.variacaoForm.value.variacoes[key].estoque_variacao;
      })
        this.produto.estoque = sum
      }
  </code></pre>
  
  <p>Everything works well when i follow this steps:</p>
  
  <ol>
  <li>Add a new form group;</li>
  <li>Fill the value in the input estoque_variacao</li>
  <li>Add other form group</li>
  <li>Fill the value of the second input estoque_variacao</li>
  <li>The sum is show correctly in the input produto.estoque</li>
  </ol>
  
  <p>The problem is when i follow this steps:</p>
  
  <blockquote>
    <ol>
    <li>Add a new Form group;</li>
    <li>Don't fill the value of the estoque_variacao of the first formGroup;</li>
    <li>Add a new form group;</li>
    <li>Try to fill the estoque_variacao of the first of second input of my form group returns NAN in my produto.estoque.</li>
    </ol>
  </blockquote>
  
  <p>My function that create the formGroup:</p>
  
  <pre><code>createFormGroup(): FormGroup {
      return this.fb.group({
           estoque_variacao: 0
  }
  </code></pre>
  
  <p>How can i fix this?</p>
  
  <p><strong>@Edit:</strong></p>
  
  <p>Worked for me:</p>
  
  <pre><code>  soma(){
      let sum = 0;
  
      const value = this.variacaoForm.value.variacoes;
  
      for(const key in value){
        sum += (+value[key].estoque_variacao);
      }
  
      this.produto.estoque = sum
    }
  </code></pre>
  
  <p>And when i remove the formgroup i need to decrease the value of my input:</p>
  
  <pre><code>this.produto.estoque = this.produto.estoque - this.variacaoForm.value.variacoes[index].estoque_variacao
  </code></pre>