Code for dolores labs tech talk.
Switch branches/tags
Nothing to show
Clone or download
Fetching latest commit…
Cannot retrieve the latest commit at this time.
Failed to load latest commit information.


Chrome Extension Development, By Example

Ben Coe (@benjamincoe)

Why Create a Chrome Extension?

  • It's a great way to integrate seamlessly with a third-party site like Gmail.
  • It in turn solves the "other site" problem, i.e., your customers don't want to go to yet another website.
  • The Chrome Web Store is a wonderful source of traffic.
  • All your friends are doing it.


  • A somewhat difficult paradigm to work with.
  • Hooking into the DOM of a 3rd party website is fragile.

Surmounting these Challenges

  • Learning to work within the paradigm.
  • Writing clean, well designed, JavaScript (make sure your DOM selectors are based on sound assumptions).
  • Listening to this presentation.

The Example, ImageMaily

ImageMaily lets you right-click on an image on a webpage and send it to an email address.

The Building Blocks

Background Pages

  • One background page runs for your extension.
  • It can make XHR requests to the endpoints specified in the manifest.
  • Can't modify the DOM of pages.

Context Menus

  • Let you add new right click options to specific types of page elements.

Content Scripts

  • Injected into every page and iframe, allow you to modify the DOM of the pages containing them.
  • The background page can be used to communicate between different content scripts, they are otherwise isolated.
  • Content scripts are sandboxed in such a way that they cannot use variables and functions defined by other scripts on the page.

A Sane Paradigm

Diagram of Design in Action

  • Message queues are a great way of thinking about designing for a Chrome extension.
  • Content Scripts pass messages to the background page.
  • The background script makes requests to the server and returns messages to content scripts.

The Important Files in this Example

  • background.js the queue based background script for handling communication with the extension.
  • content.js injected into each page, displays the jQuery-UI popup for requesting an email address.
  • server.js a Node.js script for handling XHR requests and sending the image as an email.

Creating the Context Menu Item

Background.prototype.createContextMenuItem = function() {
	var _this = this;
		type: 'normal',
		title: 'Mail Image',
		contexts: ['image'],
		onclick: function(info, tab) {
				imageUrl: info.srcUrl

The Queue-Based Paradigm

In the content script:

setInterval(function() {
	chrome.extension.sendRequest(request, function(response) {
		if (!response) return;
		if (typeof(response) == 'string') {
		} else {
			imageUrl = response.imageUrl;
			$( "#dialog-form" ).dialog( "open" );
	request = {};
}, 200);

In the background script:

// Returning data from queue.
if (typeof(_this.messages[]) == 'undefined') _this.messages[] = [];
	if (_this.messages[].length > 0) {
	} else {
// Populating the queue from an XHR request.
Background.prototype.sendEmail = function(request, tab) {
	var _this = this;
	if (typeof(_this.messages[]) == 'undefined') _this.messages[] = [];
		type: 'get',
		url: 'http://localhost',
		data: {
			imageUrl: request.imageUrl,
		success: function(response) {
			if (response.success) {
				_this.messages[].push( _this.successMessage );
			} else {
				_this.messages[].push( _this.failureMessage );
		error: function() {
			_this.messages[].push( _this.failureMessage );

The Server

The server, written in Node.js, handles downloading the image and emailing it.

app.get('/', function(req, res){
	var imageUrl = req.param('imageUrl'),
		filename = ( imageUrl.match(/^.*\/([^/]*)$/)[1] ).match(/([^?&]*)/)[1];
	downloadImage(imageUrl, function(imageData) {
		sendMail(imageData, filename, req.param('email'), function(err, success) {
			if (err) {
					success: false
			} else {
					success: true


If approached in a sane, methodical way Chrome Extension development can be a fun paradigm to work within. Building extensions is a great way to get more users for your SaaS offering.