The $$ PREFIX is used to define a private variable in AngularJS. This is responsible for avoiding accidental code changes due to exposure to the variables. Examples are $$observers, $$watchers, $$childScope etc.
The $ prefix is used for defining built-in core AngularJS FUNCTIONALITIES like variable, parameter, method, or any properties. Examples are $scope, $http, $routeProvider, $watch etc.

Conclusion:
In this ARTICLE, we have seen the most commonly asked AngularJS interview questions for both the freshers and. the experienced developers. AngularJS was initially created to establish the concept of dependency injection and ease the development of single-page APPLICATIONS. Despite AngularJS being the first version, many companies have adopted this framework due to its raging benefits, easy configuration, and extensibility. Even though the framework has undergone a lot of changes in its different versions and releases, AngularJS has been the most useful framework for developing web applications in different companies.

References: 

https://angularjs.org/
https://thinkster.io/a-better-way-to-learn-angularjs

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