Explain what is injector?

Answer 1

Injector:

$injector is used to retrieve object instances as defined by provider, instantiate types, invoke methods, and load modules.

The following always holds true:

var $injector = angular.injector();
expect($injector.get('$injector')).toBe($injector);
expect($injector.invoke(function($injector) {
  return $injector;
})).toBe($injector);