I'm new to angularJS and want to implement the efficient thing for my project but got stuck between $onInit (life cycle hook) and activate().
Creating an activate() function inside of your controller and calling it directly is quite different than using the $onInit() lifecycle hook provided by AngularJS.
From https://docs.angularjs.org/guide/component#component-based-application-architecture:
$onInit()- Called on each controller after all the controllers on an element have been constructed and had their bindings initialized (and before the pre & post linking functions for the directives on this element). This is a good place to put initialization code for your controller.
So basically the activate() function will be called as soon as your controller is constructed. Where as the $onInit() function will be called after all bindings have be successfully bound. Thus if you try to access your bound variables within your constructor, they will not be initialized yet.