title: Angular 5 formgroup list controls or iterate them
body: <p>I am using angular 5 with formgorup and wish to iterate the controls, in order to create a dynamic component based on a form, the forms fields are given by an external data service (database etc.) </p>
  
  <p>It is declares as follows </p>
  
  <pre><code>check = new FormGroup({
      firstName : new FormControl(true),
      lastName : new FormControl(false)
    });
  </code></pre>
  
  <p>I found <a href="https://stackoverflow.com/questions/42235156/angular-2-iterate-over-reactive-form-controls">this</a> explaining how to iterate the controls but It does not work. I try to use:</p>
  
  <pre><code>for(let item of this.check.controls){}
  </code></pre>
  
  <p>and get this is the chrome debug: </p>
  
  <blockquote>
    <p>Cannot read property 'length' of undefined</p>
  </blockquote>
  
  <p>I can't access this.check.controls.keys or keys() </p>
  
  <p>How can I iterate the keys? </p>
  
  <p>Thanks</p>