For questions about ngoninit, a lifecycle hook that is called after Angular has initialized all data-bound properties of a directive.
ngOnInit is a callback method that is invoked immediately after the default change detector has checked the directive's data-bound properties for the first time, and before any of the view or content children have been checked. It is invoked only once when the directive is instantiated to handle any additional initialization tasks.
See also, the documentation for the interface OnInit
