this.profileForm = this.formBuilder.group({
  firstName:['', Validators.required],
  lastName:['', Validators.required]
});

this.profileForm = this.formBuilder.group({
  firstName:[''],
  lastName:['']
}, [Validators.required]);

this.profileForm 
= this.formBuilder.group({
  firstName: new FormControl(''),
  lastName: new FormControl('')
}), [Validators.required]);

this.profileForm= this.formBuilder.group({
  firstName: new FormRequiredControl(''),
  lastName: new FormRequiredControl('')
}), [Validators.required]);