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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.