Angular 2 Constructors:-
1. The constructor is a default method runs when component is being constructed.
2. The constructor is a typescript feature and it is used only for a class instantiations and nothing to do with Angular 2.
3. The constructor called first time before the ngOnInit().
Stayed Informed - 13 Best Advantages for Angular 2
Angular 2 ngOnInit:-
1. The ngOnInit event is an Angular 2 life-cycle event method that is called after the first ngOnChanges and the ngOnInit method is use to parameters defined with @Input otherwise the constructor is OK.
2. The ngOnInit is called after the constructor and ngOnInit is called after the first ngOnChanges.
3. The ngOnChanges is called when an input or output binding value changes.