Why should I create my Angular2 reactive form in the constructor instead of ngOnInit?

Question

As indicated in other responses, the initial routines of an Angular2 application should be launched in the ngOnInit() method, leaving the constructor specifically for dependency injection.

However, in the Reactive Forms tutorial that I'm following, the initialization of the form is in the constructor:

export class HeroDetailComponent3 {
  heroForm: FormGroup; // <--- heroForm is of type FormGroup

  constructor(private fb: FormBuilder) { // <--- inject FormBuilder
    this.createForm();
  }

  createForm() {
    this.heroForm = this.fb.group({
      name: '', // <--- the FormControl called "name"
    });
  }
}

Is there really a significant difference or is it just a minor issue?

Answer 1

Initializing the formGroup in ngOnInit() is not a bad practice, as it will actually be required if you want your form to be initialized with values that depend (directly or indirectly) from your component @Input()s.

For instance:

class SignInFormComponent {
  @Input() currentLogin: string;
  formGroup: FormGroup;

  constructor(private fb: FormBuilder) {
    // this.currentLogin is not known yet here
  }

  ngOnInit(): void {
    this.formGroup = this.fb.group({
      loginEmail: [this.currentLogin, Validators.email],
      loginPassword: [''],
    });
  }
}

Answer 2

Depends upon your specific use case and design. Possibly you will run into issues relating to get methods on which some of your other formControls might depend on. You might benefit from creating your form inside the constructor and your form will be ready as soon as the component is rendered. However, if you need to do subscribe to .valueChanges on the form for some formControls, you might have a timing issue. You cannot build your dependent formControls unless you .subscribe, and you cannot .subscribe until the form is initialized. You might encounter error: cannot read property 'get' of undefined.

Answer 3

I belive it because of the createForm method in constructor will execute before ngOninit and your form will be ready for use as soon your component is rendered.