Chrome Extensions for Web Hackers

Chrome Extensions
for Web Hackers
Scandinavian Web Developer Conference 2010, Stockholm, Sweden
Mark Wubben

A talk about Chrome Extensions, why they’re so great for web hackers and how to build
them.

Given at the Scandinavian Web Developer Conference on June 2nd, 2010 in Stockholm,
Sweden.

Licensed under Creative Commons Attribution-Share Alike 2.5
http://creativecommons.org/licenses/by-sa/2.5/dk/

Photo by Steve Peck, http://www.ﬂickr.com/photos/stevepeck/4615314670/. CC-BY-2.0.

I’m a Web Hacker
I’m a Web Hacker. That might scare some people, but I don’t hack highway signs or break
into computers. I (try to) build cool shit.

I think you’re web hackers as well, otherwise, why would you be here? But if you’re a web
hacker, why limit yourself to building websites? Why not hack other systems? Build, say,
browser extensions?

Photo by James Kim, http://www.ﬂickr.com/photos/underbiteman/2638246638/. CC-
BY-2.0.

Add-ons, Plugins, Extensions
My question to you is, have you ever build a browser extension? Did you try? Did you look
into it and got scared? That’s been my story—Firefox extension have always looked too
bewildering. You’d have to know about RDF, XUL, XPCOM. You had to restart your browser
every time you wanted to try something. Bah!

Now, this talk is about Google Chrome. And this time, it’s different.

Photo by Jon Fife, http://www.ﬂickr.com/photos/good-karma/652486713/. CC-BY-SA-2.0.

Open Web Technology
See, Chrome’s Extension platform is based on Open Web Technology. It’s based on
JavaScript, mostly. But also HTML and CSS. And the cool new HTML5ish APIs like localStorage
or geo-location.

Let’s dive in.

Photo by Kevin Dooley, http://www.ﬂickr.com/photos/pagedooley/4126491780/. CC-
BY-2.0.

This is the Extensions page in Chrome. It shows you which extensions you have installed. You
can disable or uninstall them, see their web page, allow them to run in Incognito mode.

At the bottom is a link to the Google Chrome Extensions Gallery. At the top is an option to
enable Developer mode.

Developer mode lets you load a new extension from your local ﬁle system. You can also
package extensions for release or force the installed extensions to be updated. Normally this
is done automatically when the browser is restarted.

So, what is an extension?!

Now the obvious question is, what *is* an extension?!

An extension is a folder
with a manifest.json file

In essence, an extension isn’t much more than a folder that contains a manifest.json ﬁle.

Let’s try loading a few folders as an extension.

{
"name": "SWDC For The Win!",
"version": "1.0"
}

manifest.json
And there’s the manifest for our very simple extension. These are the only two required
properties, a name for your extension and a version number.

Admittedly, this extension doesn’t do much.

For more about the manifest ﬁle, see http://code.google.com/chrome/extensions/
manifest.html.

Extensions can have
content scripts.

One of the things a Chrome Extension can do is to run scripts on the pages you visit. These
are content scripts and should be familiar, because its a concept from the Firefox-based
Greasemonkey add-on.

Aaron Boodman / Greasemonkey
In fact, the guy who invented Greasemonkey, Aaron Boodman, has been at Google for a few
years now and is one of the guys behind the new Extensions platform. To put it differently,
Chrome Extensions is Greasemonkey on steroids.

Photo by Mark Wubben, http://www.ﬂickr.com/photos/novemberborn/230693761/. CC-BY-
SA-2.0.

Fixing Twitter opening
links in new windows

You might have noticed how external links on Twitter always open in a new window. I find
this annoying, so I figured I’d write an extension to fix it.

{
"name": "Twitter Fixer",
"version": "1.0",
"description": "Fix the external links…",
"content_scripts": [{
"matches": ["http://*.twitter.com/*",
"https://*.twitter.com/*"],
"js": ["dojo.js", "fixer.js"]
}]
}

manifest.json
The manifest for our new extension, dubbed Twitter Fixer.

{
"version": "1.0",
}]
}

manifest.json
Note how I’ve added a description.

{
"version": "1.0",
}]
}

manifest.json
You can specify multiple content scripts per extension.

{
"version": "1.0",
}]
}

manifest.json
A content scripts needs to match a page. This is done through match patterns. Here we
specify our extension will run on any page or subdomain on twitter.com, over HTTP as well as
HTTPS.

Keep in mind that the user is warned about the sites you might match. The more restrictive
your match pattern, the better.

To learn more, see http://code.google.com/chrome/extensions/match_patterns.html.

{
"version": "1.0",
}]
}

manifest.json
A content script itself can exist of any number of JavaScript ﬁles. They’re loaded in the same
order as you specify in the manifest. Here I load a version of the Dojo Toolkit and my own
code.

You can also specify CSS ﬁles that need to be added to the page your content script runs on.

To learn more, see http://code.google.com/chrome/extensions/content_scripts.html.

dojo.query("a[target=_blank]").attr("target", "");

fixer.js
A content script itself can exist of any number of JavaScript ﬁles. They’re loaded in the same
order as you specify in the manifest. Here I load a version of the Dojo Toolkit and my own
code.

You can also specify CSS ﬁles that need to be added to the page your content script runs on.

To learn more, see http://code.google.com/chrome/extensions/content_scripts.html.

Isolated Worlds
Chrome has learned from the security problems that existed with Greasemonkey, and even
with Firefox add-ons as a whole. Each extension lives in a so-called “isolated world”,
meaning it’s isolated from other extensions save for a few tightly controlled communication
bridges.

Photo by F.H. Mira, http://www.ﬂickr.com/photos/fhmira/3204656258/sizes/o/. CC-BY-
SA-2.0.

Content scripts run in
separate contexts

For example, the JavaScript inside your content scripts is evaluated in a separate context
from the page JavaScript. This means your code won’t affect the page code, and vice versa.
You can’t directly call page code, and it can’t directly call your code.

Shared DOM

Luckily the page document is shared between the various content scripts that might be
running on it. That way, you can change it!

Communicating with page
JavaScript

But with these isolated worlds, how can your content scripts talk to the page JavaScript? Well,
you’ve got access to the DOM, so you can insert your own JavaScript into the page! And, you
can use DOM events so the inserted JavaScript can talk back to you.

document.documentElement.addEventListener(
"SWDCNotify",
function(){ alert("Notified!"); },
false
);

var s = document.createElement("script");
s.textContent = 'function notifyContentScript(){
var evt = document.createEvent("Event");
evt.initEvent("SWDCNotify", false, false);
document.documentElement.dispatchEvent(evt);
}';
document.body.appendChild(s);

communication.js
This sets up a content script that insert the `notifyContentScript` method into the page.
When this method is called, a custom DOM event is dispatched on the document element,
which is used to notify the content script.

While you can’t send data along with the event, you can store it in the DOM. The content
script can then look it up.

document.documentElement.addEventListener(
"SWDCNotify",
function(){ alert("Notified!"); },
false
);

var s = document.createElement("script");
s.textContent = 'function notifyContentScript(){
var evt = document.createEvent("Event");
evt.initEvent("SWDCNotify", false, false);
document.documentElement.dispatchEvent(evt);
}';
document.body.appendChild(s);

communication.js

Content scripts are limited.
Background pages!

Content scripts are fairly limited though. They exist only as long as the page they run on
exists. They don’t have access to any permanent storage, so you can’t conﬁgure them. Nor
can they talk to other websites, so you can’t look up anything through an API.

Luckily, Chrome Extensions let you build background pages. These are normal HTML pages,
except that they’re not rendered. They’re loaded when the browser starts, and won’t be
unloaded until it’s closed.

Let’s build a more complicated extension.

Expanding bit.ly URLs on
Twitter

Due to character constraints URLs in Tweets are often shortened. But, I’d like to see where
I’m going! Let’s write Chrome Extension that can expand bit.ly URLs.

{
"version": "1.0",
"description": "Expands shortened URLs…",
"permissions": ["http://api.bit.ly/*"],
"background_page": "background.html",
"run_at": "document_end",
}]
}

manifest.json

{
"version": "1.0",
}]
}

manifest.json
I’ve made two major modiﬁcations to the manifest.json we used previously. First is loading
the background page, this is done using the background_page property whose value is the
relative path (from the manifest.json ﬁle) to the background page. By convention this is
named background.html, but you can name it whatever you like.

The other change is that I’m now requesting permission to talk to the Bit.ly API. Chrome
forces the extension developer to request permission for almost anything. When the user
installs your extension he’s made aware of what you’re extension will have permission to,
therefore making it harder for nefarious Extension developers to sneak bad stuff into their
extensions without the users knowing about it.

{
"version": "1.0",
}]
}

manifest.json
Another change I made is to specify the `run_at` property for the content script. This way I
can make sure it runs right after the page document has ﬁnished parsing, so we don’t have
to wait too long before we can expand the bit.ly URLs.

parsed.hashes,
function(mapping){
});
}
}
);

fixer.js
The content script needs to talk to the background page to expand the hashes. This is done
through the `chrome.extension.sendRequest` API.

parsed.hashes,
function(mapping){
});
}
}
);

fixer.js
Also note how I can use forEach on an array. Chrome has a fully up-to-date JavaScript engine
so native forEach is available.

Same for using textContent to set the link text value.

<!DOCTYPE html>

<script src="dojo.js"></script>
<script>
chrome.extension.onRequest.addListener(
function(hashes, sender, sendResponse){
// …
sendResponse(mapping);
}
);
</script>

background.html
I won’t go into the speciﬁcs of how to talk to bit.ly and get the expanded URLs.

<!DOCTYPE html>

<script src="dojo.js"></script>
<script>
chrome.extension.onRequest.addListener(
function(hashes, sender, sendResponse){
// …
sendResponse(mapping);
}
);
</script>

background.html
The important bit is how you register a handler for requests from the content script. You get
the request object, a reference to the tab from which the request was sent, and a callback
function to call once you have a response.

This communication mechanism is completely asynchronous.

Debugging
Web Inspector

You can easily debug your extension using the Web Inspector you would normally use to
debug your web pages. You can set break points or use the debugger keyword. An inspector
for the background page can be opened by clicking on the “background.html” link in the
Extensions page (if you have Developer mode enabled).

You may also notice how the background page is actually at “chrome-extension://some-
unique-identiﬁer/background.html”. This is the domain it runs in, so the extension can
piggyback on the normal same-origin restrictions!

Extension configuration,
persistent storage

When talking about the limitations of content scripts, I promised you permanent storage and
extension conﬁguration. Let’s have a look how that’s done.

Combining the extensions,
with options

Let’s combine the two extensions we’ve made into a single extension. Since not everybody
wants the external links on Twitter to be opened in a new window, we’ll add an options page
so this can be customized.

{
"version": "1.0",
"description": "Fix the external links …",
"options_page": "options.html",
}]
}

manifest.json
Our new manifest.

{
"version": "1.0",
"description": "Fix the external links …",
"options_page": "options.html",
}]
}

manifest.json
Chrome has special support for an Options page. It’ll show up under the extension name
when it’s listed in the Extension page, and simply is a HTML page you provide.

I won’t show the changes made to combine the extensions, but in general, the content script
now talks to the background page to see which feature is enabled. The background page
looks it up in localStorage, which is where the options page has saved the conﬁguration.

Chrome APIs

Chrome provides a ton of proprietary APIs for interacting with the user. A quick overview.

Page actions
Browser actions
Popups

You can add an action button to the browser Chrome. If the action button is speciﬁc to a
page, you can use a Page action. This can be controlled to only show up when it’s necessary.
Browser actions are always visible. An extension cannot specify both a page and a browser
action.

You can use an image as the action icon, or use output from the Canvas API. Browser actions
can have badges that communicate some information, say an unread messages count.

When the user clicks on a page/browser action, you can show a tooltip.

Desktop Notifications

You can show Desktop Notiﬁcations, provided you requested permission to do this.

History
Bookmarks

You can also interact with the browser history and the user’s bookmarks. Plus, you can
override the History page to provide a better interface.

Tabs
Windows

Chrome also lets you manipulate open tabs and windows. You can override the new tab page.

You can’t override the Extensions page though, for obvious reasons.

Publishing

But, aside from all these capabilities, perhaps the most important thing is having other
people use your extension!

So far we’ve loaded extensions from your local machine. Let’s see how we can package those
local ﬁles so that anybody can install your extension.

Demo time

You can pack your extension using the “Pack extension…” option on the Extensions page (in
Developer mode). This takes the path to your extension and a possible private key file.

The end result is a `.crx` file, which in essence is a signed ZIP file. You’ll also get a `.pem`
file which contains the private key for your extension. This is important, the private key is
used to sign the ZIP file, and Chrome will refuse to update your extension if the new
extension file was not signed with the same private key.

Gallery

Chrome also has a Gallery where you can distribute your extension. They manage packaging
and updating your extension, and will also keep track of the private key for you. This of
course is the best place for your extension, though you’re free to host it yourself.

In the latter case, you’ll have to set up some extra ﬁles on your server to support auto-
updating of your extension.

Questions?

Mark Wubben

supercollider.dk & novemberborn.net
twitter.com/novemberborn

Slides: 11born.net/swdc

Licensed under Creative Commons Attribution-Share Alike 2.5
http://creativecommons.org/licenses/by-sa/2.5/dk/

And that concludes this talk. Thank you for your attention.

Steve Peck
James Kim
Jon Fife
Kevin Dooley
F H Mira
Matt Jones
Jeff Kubina

Many, many thanks to the wonderful people on Flickr who licensed their photos under
Creative Commons.

Photo by Jeff Kubina, http://ﬂickr.com/photos/kubina/903033693/. CC-BY-SA 2.0.

Now, as Matt Jones would put it, GET EXCITED and MAKE THINGS!

Illustration by Matt Jones, CC-BY-SA-NC, http://www.ﬂickr.com/photos/blackbeltjones/
3365682994/.

Chrome Extensions for Web Hackers

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