Lifecycle Hooks Angular 2 docs
(From halfway down the page) We turn to
ngOnInit for two main reasons:
- To perform complex initializations shortly after construction
- To set up the component after Angular sets the input properties
HTTP client – oninit Angular 2 docs
Although at runtime the component requests heroes immediately after creation, we do not call the service’s
getmethod in the component’s constructor. We call it inside the
ngOnInit lifecycle hook instead and count on Angular to call
ngOnInit when it instantiates this component.
This is a best practice. Components are easier to test and debug when their constructors are simple and all real work (especially calling a remote server) is handled in a separate method.