title: Angular 2+ removeAt function not working in child component
body: <p>I'm using for model driven for my form in angular4 .</p>
  
  <p>I pass a <code>formarray</code> to child component by <code>@input</code> and when i use removeAt to it catch me an error :</p>
  
  <blockquote>
    <p>removeAt is not a function</p>
  </blockquote>
  
  <p>my parent component.html</p>
  
  <pre><code>&lt;form class="ui form" [formGroup]="gform" (ngSubmit)="doSave(gform.value)"&gt;
  &lt;app-tag-input [_input]="spectra.controls"  &gt;&lt;/app-tag-input&gt;
  &lt;/form&gt;
  </code></pre>
  
  <p>parentcoponent.ts</p>
  
  <pre><code>     gform: FormGroup;
         get spectra(): FormArray { return this.gform.get('spectra') as 
         FormArray; }
         ngOnInit() {
          this.spectra.insert(0, this.initSpectrum(' very good'));
              this.spectra.insert(1, this.initSpectrum('good'));
              this.spectra.insert(2, this.initSpectrum('normal'));
              this.spectra.insert(3, this.initSpectrum('need to more try'));}
  </code></pre>
  
  <p>childcoponent.ts:</p>
  
  <pre><code>export class TagInputComponent implements OnInit {
  
  @Input() _input :FormArray;
  tagEntry:string;
  
    constructor(private formBuilder:FormBuilder) {
      formBuilder.array
      }
    addToInput() {
      const formGroup=this.formBuilder.control(
        this.tagEntry
      );
      const order = this._input.length + 1;
      this._input.push(formGroup)
      this.tagEntry='';
  
    }
    removeSpectrum=(i: number)=&gt; {
  
      const control = &lt;FormArray&gt;this._input;
  
      control.removeAt(i);
    }
    ngOnInit() {
  
    }
  
  }
  </code></pre>
  
  <p>my child component.html</p>
  
  <pre><code>&lt;div class="tagsinput"&gt;
    &lt;span *ngFor="let item of _input let i=index" class="ui teal  label"&gt;
      {{item.value}}
      &lt;i class="close icon" (click)="removeSpectrum(i)"&gt;&lt;/i&gt;
  
    &lt;/span&gt;
    &lt;input type="text"  [(ngModel)]="tagEntry"  [ngModelOptions]="{standalone: true}" (keyup.enter)="addToInput()"/&gt;
  &lt;/div&gt;
  </code></pre>
  
  <p>when i console formarray in tow component an control object exist in parent component <code>spectra</code> that not in _input in child component.</p>