In node if you require the events module you can use the so-called 'event emitter' that node itself uses for all of its APIs that emit things.
Events are a common pattern in programming, known more widely as the 'observer pattern' or 'pub/sub' (publish/subscribe). Whereas callbacks are a one-to-one relationship between the thing waiting for the callback and the thing calling the callback, events are the same exact pattern except with a many-to-many API.
The easiest way to think about events is that they let you subscribe to things. You can say 'when X do Y', whereas with plain callbacks it is 'do X then Y'.
Here are few common use cases for using events instead of plain callbacks:
- Chat room where you want to broadcast messages to many listeners
- Game server that needs to know when new players connect, disconnect, move, shoot and jump
- Game engine where you want to let game developers subscribe to events like .on('jump', function() {})
- A low level web server that wants to expose an API to easily hook into events that happen like .on('incomingRequest') or .on('serverError')
by Max Odgen in his art-of-node
Although in most of the lessons you will be given already created EventEmitter, you may need to require events library (don't worry, it's built in).
Then, you can initiate your own EventEmitter by simply doing the following:
var EventEmitter = require('events').EventEmitter;
var emitter = new EventEmitter();You are given an EventEmitter called phone (see the boilerplate below). Subscribe to a call event and print a message Ma phone is calling! every time that event occurs.
// emitter is an EventEmitter instance that will emit some events as per Task description
module.exports = function(phone) {
};