OnInit and ngOnInit

OnInit  ( interface OnInit )

A lifecycle hook that is called after Angular has initialized all data-bound properties of a directive. Define an ngOnInit() method to handle any additional initialization tasks.

export class ProductAlertsComponent implements OnInit { }

ngOnInit()

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.

export class ProductAlertsComponent implements OnInit {

  ngOnInit() {   }

}

Ref: angular.io