The constructor is a typescript feature. The constructor
is only related to class instantiation and it’s nothing to do with Angular 2 and
it is use to some initialization processing with respect to class hierarchies for
the newly created instance.
The ngOnInit event is an Angular 2 life-cycle
event/ method that are called after the first ngOnChanges. The ngOnInit method
is use to parameters defined with @Input otherwise the constructor is OK.