Working with Events

The Add-on SDK supports event-driven programming through its EventEmitter framework.

Objects emit events on state changes that might be of interest to add-on code, such as browser windows opening, pages loading, network requests completing, and mouse clicks. By registering a listener function to an event emitter an add-on can receive notifications of these events.

We talk about content scripts in more detail in the Working with Content Scripts guide.

Additionally, if you're using content scripts to interact with web content, you can define your own events and use them to communicate between the main add-on code and the content scripts. In this case one end of the conversation emits the events, and the other end listens to them.

So there are two main ways you will interact with the EventEmitter framework:

  • listening to built-in events emitted by objects in the SDK, such as tabs opening, pages loading, mouse clicks

  • sending and receiving user-defined events between content scripts and add-on code

This guide only covers the first of these; the second is explained in the Working with Content Scripts guide.

Adding Listeners

You can add a listener to an event emitter by calling its on(type, listener) method.

It takes two parameters:

  • type: the type of event we are interested in, identified by a string. Many event emitters may emit more than one type of event: for example, a browser window might emit both open and close events. The list of valid event types is specific to an event emitter and is included with its documentation.

  • listener: the listener itself. This is a function which will be called whenever the event occurs. The arguments that will be passed to the listener are specific to an event type and are documented with the event emitter.

For example, the following add-on registers two listeners with the private-browsing module to listen for the start and stop events, and logs a string to the console reporting the change:

var pb = require("private-browsing");

pb.on("start", function() {
  console.log("Private browsing is on");

pb.on("stop", function() {
  console.log("Private browsing is off");

It is not possible to enumerate the set of listeners for a given event.

The value of this in the listener function is the object that emitted the event.

Adding Listeners in Constructors

Event emitters may be modules, as is the case for the private-browsing events, or they may be objects returned by constructors.

In the latter case the options object passed to the constructor typically defines properties whose names are the names of supported event types prefixed with "on": for example, "onOpen", "onReady" and so on. Then in the constructor you can assign a listener function to this property as an alternative to calling the object's on() method.

For example: the widget object emits an event when the widget is clicked.

The following add-on creates a widget and assigns a listener to the onClick property of the options object supplied to the widget's constructor. The listener loads the Google home page:

var widgets = require("widget");
var tabs = require("tabs");

  id: "google-link",
  label: "Widget with an image and a click handler",
  contentURL: "",
  onClick: function() {"");

This is exactly equivalent to constructing the widget and then calling the widget's on() method:

var widgets = require("widget");
var tabs = require("tabs");

var widget = widgets.Widget({
  id: "google-link-alternative",
  label: "Widget with an image and a click handler",
  contentURL: ""

widget.on("click", function() {"");

Removing Event Listeners

Event listeners can be removed by calling removeListener(type, listener), supplying the type of event and the listener to remove.

The listener must have been previously been added using one of the methods described above.

In the following add-on, we add two listeners to private-browsing's start event, enter and exit private browsing, then remove the first listener and enter private browsing again.

var pb = require("private-browsing");

function listener1() {
  console.log("Listener 1");
  pb.removeListener("start", listener1);

function listener2() {
  console.log("Listener 2");

pb.on("start", listener1);
pb.on("start", listener2);


Removing listeners is optional since they will be removed in any case when the application or add-on is unloaded.