Chapter 5

Using the Event Emitter Pattern to Simplify Event Binding

WHAT’S IN THIS CHAPTER?

  • Introducing the event emitter pattern
  • Binding and unbinding event listeners
  • Creating your own event emitter

In Node many objects emit events. For instance, a TCP server can emit a “connect” event every time a new client connects, or a file stream can emit a “data” event every time a new chunk of data is read. These objects are, in Node nomenclature, event emitters. Event emitters allow programmers to subscribe to events they are interested in. The programmer attaches a callback function that will be invoked every time a relevant event in that event emitter occurs. This publisher/subscriber pattern is very similar to the typical GUI pattern, whereby a program gets notified that a certain button was clicked. By using this pattern, a server-side program can react when, for instance, a client connects to the server, data is available on a socket, or a file gets closed.

You can also create your own event emitters. In fact, Node supplies an EventEmitter pseudo-class that can work as a base for creating your own event emitters.

UNDERSTANDING THE STANDARD CALLBACK PATTERN

Asynchronous programming does not use function return values to denote that a function is finished. Instead it uses the continuation-passing style (CPS):

Continuation-passing style (CPS) is a style of programming in which control is passed explicitly in the form of a continuation. (...)

A function written in continuation-passing ...

Get Professional Node.js: Building Javascript Based Scalable Software now with the O’Reilly learning platform.

O’Reilly members experience books, live events, courses curated by job role, and more from O’Reilly and nearly 200 top publishers.