Why do we use $rootScope.$broadcast in AngularJS?

$rootScope basically functions as an event listener and dispatcher.

To answer the question of how it is used, it used in conjunction with rootScope.$on;

$rootScope.$broadcast("hi");

$rootScope.$on("hi", function(){
    //do something
});

However, it is a bad practice to use $rootScope as your own app's general event service, since you will quickly end up in a situation where every app depends on $rootScope, and you do not know what components are listening to what events.

The best practice is to create a service for each custom event you want to listen to or broadcast.

.service("hiEventService",function($rootScope) {
    this.broadcast = function() {$rootScope.$broadcast("hi")}
    this.listen = function(callback) {$rootScope.$on("hi",callback)}
})

1. What does $rootScope.$broadcast do?

   $rootScope.$broadcast is sending an event through the application scope. Any children scope of that app can catch it using a simple: $scope.$on().

   It is especially useful to send events when you want to reach a scope that is not a direct parent (A branch of a parent for example)

   !!! One thing to not do however is to use $rootScope.$on from a controller. $rootScope is the application, when your controller is destroyed that event listener will still exist, and when your controller will be created again, it will just pile up more event listeners. (So one broadcast will be caught multiple times). Use $scope.$on() instead, and the listeners will also get destroyed.

2. What is the difference between $rootScope.$broadcast & $rootScope.$broadcast.apply?

   Sometimes you have to use apply(), especially when working with directives and other JS libraries. However since I don't know that code base, I wouldn't be able to tell if that's the case here.