How AngularJS prefixes $ and $$ are used?

• $:

  The $ in AngularJs is a built-in object.It contains application data and methods.

  The scope($) acts as a link between controller and view.

  👁 Image
  

  Inside the controller function the properties and the methods to the scope($) can be attached. Expression, ng-model, or ng-bind directive can be used to display the scope data in the view.

  Output: 👁 Image
  Example 2: Output: 👁 Image
  

  $rootScope:

  The AngularJS application consists of a single $rootScope. All the other $scope are child objects.$rootscope has properties and methods attached to it which will be available be to all the controllers.

  Method	Description
  $new	It is used to create new Child Scope
  $watch	It is used to Register a callback which is to be executed whenever the model property changes.
  $watchGroup	It is used to register a callback  which is to be executed whenever model properties changes. We specify an array of properties in this.
  $watchCollection	It is used to register a callback which is to be executed whenever model object or array property changes.
  $digest	It processes all of the watchers of the current scope and its children.
  $destroy	Remove the current scope from parent scope.
  $eval	Execute expressions on the current scope.
  $emit	It is used to dispatch the specified event upwards till $rootScope.
  $broadcast	It is used to dispatch the specified event downwards till child Scope.

  Example 3: Output: 👁 Image
  

• $$ in this are treated as private variables. We use $$ to avoid the internal variable conflicts and not to expose for external use.

  Some of them are listed below:-

  $$observers, $$watchers, $$childHead, $$childTail, $$ChildScope etc.

  Method	Description
  $$watchers	It contains all of the watches associated with the scope
  $$asyncQueue	It is a async task queue.It is consumed on every digest
  $$postDigest(fn)	It executes fn after the next digest cycle.
  $$destroyed	It destroys the scope.

  Syntax:

  $$('.selector');

  or

  element.all(by.css('.selector'));

  A common ways of communication between modules of application, with the help of core AngularJS functionality is establishing a connection between controllers using $parent, $$childHead, $$nextSibling.

  Example: Output: 👁 Image
  

  The above output will be produced when we add $$, since, it acts as private object.

So, to prevent accidental name collisions while writing your code angular prefixes public objects with $ and private objects with $$.