Now, Unlike Angular 1, Angular 5 does not have a bi-directional digest cycle. In this version, any changes occurring within the component generally get propagated to all the children components present in the hierarchy through the current component.
If there is any change in one component which needs to be reflected in its parent component in the respective hierarchy, you can easily emit an event using the Event Emitter API which is native from Angular 5.
So basically, EventEmitter is defined via a class in the @angular/core module which is used equally by components and directives for emitting any custom events.

BY Best Interview Question ON 13 May 2020