Node.js core API is based on asynchronous event-driven architecture in which certain kind of objects called emitters periodically emit events that cause listener objects to be called.

All objects that emit events are members of EventEmitter class. These objects expose an eventEmitter.on() function that allows one or more functions to be attached to named events emitted by the object.

When the EventEmitter object emits an event, all of the functions attached to that specific event are called synchronously. All values returned by the called listeners are ignored and will be discarded.

Consider the following example: