Angular 5 formgroup list controls or iterate them

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.)

It is declares as follows

check = new FormGroup({
    firstName : new FormControl(true),
    lastName : new FormControl(false)
  });

I found this explaining how to iterate the controls but It does not work. I try to use:

for(let item of this.check.controls){}

and get this is the chrome debug:

Cannot read property 'length' of undefined

I can't access this.check.controls.keys or keys()

How can I iterate the keys?

Thanks