title: Reactive Forms: dynamically add and delete to a type string array in Angular 4
body: <p>How do I add a field to a reactive form that can dynamically add and delete to a type string array? The implementation I have right now is a FormArray, however it adds to the array as a object rather that just another string in the array. (see code below)</p>
  
  <p><div class="snippet" data-lang="js" data-hide="false" data-console="true" data-babel="false">
  <div class="snippet-code">
  <pre class="snippet-code-js lang-js prettyprint-override"><code>ngOnInit() {
    this.form = this.formBuilder.group({
      user: '',
      directory: '',
      filename: this.formBuilder.array([ this.createItem() ])
    });
  }
  
  createItem(): FormGroup {
    return this.formBuilder.group({
      filename: ''
    });
  }</code></pre>
  </div>
  </div>
  </p>