Where to initialise a FormGroup in Reactive forms?

Shouldn't the FormGroup be initialised on ngOnInit?

It doesn't have to be; I find it simplest to initialize most things directly on the class like this:

export class ContactComponent implements OnInit { 

  form = this.formBuilder.group({ 
    email: ['', [Validators.required, Validators.email]],
    name:  ['', [Validators.required, Validators.maxLength(40)]],
  });

  constructor(private formBuilder: FormBuilder) { }

}

This works fine with strictPropertyInitialization.

The exception is when initializing something that requires data from the DOM (@Input, @ViewChild, etc) Since those things get initialized in the various Lifecycle Hooks as described in this answer:

• @Input() → ngOnInit
• @ViewChild() → ngAfterViewInit
• @ContentChildren() → ngAfterContentInit

Initializing something that depends on say, an @Input(), would be done in the corresponding lifecycle hook, ngOnInit, however, this does not appease the strict rule, so the Definite Assignment Assertion Operator ! is needed.

export class ContactComponent implements OnInit { 
  @Input() input!: number;

  inputDoubled!: number;
  
  form = this.formBuilder.group({ 
    email: ['', [Validators.required, Validators.email]],
    name:  ['', [Validators.required, Validators.maxLength(40)]],
  });

  constructor(private formBuilder: FormBuilder) { }

  ngOnInit() {
    this.inputDoubled = this.input * 2;
  }
}