Getting Started: Building a Chrome Extension

Extensions allow you to add functionality to Chrome without diving deeply into native code. You can create new extensions for Chrome with those core technologies that you're already familiar with from web development: HTML, CSS, and JavaScript. If you've ever built a web page, you should feel right at home with extensions pretty quickly; we'll put that to the test right now by walking through the construction of a simple extension that will give you one-click access to pictures of kittens. Kittens!

We'll do so by implementing a UI element we call a browser action, which allows us to place a clickable icon right next to Chrome's Omnibox for easy access. Clicking that icon will open a popup window filled with kittenish goodness, which will look something like this:

Chrome, with an extension's popup open and displaying many kittens.

If you'd like to follow along at home (and you should!), create a shiny new directory on your computer, and pop open your favourite text editor. Let's get going!

Something to Declare

The very first thing we'll need to create is a manifest file named manifest.json. The manifest is nothing more than a JSON-formatted table of contents, containing properties like your extension's name and description, its version number, and so on. At a high level, we'll use it to declare to Chrome what the extension is going to do, and what permissions it requires in order to do those things.

In order to display kittens, we'll want to tell Chrome that we'd like to create a browser action, and that we'd like free-reign to access kittens from a particular source on the net. A manifest file containing those instructions looks like this: 

{
  "manifest_version": 2,

  "name": "One-click Kittens",
  "description": "This extension demonstrates a browser action with kittens.",
  "version": "1.0",

  "permissions": [
    "https://secure.flickr.com/"
  ],
  "browser_action": {
    "default_icon": "icon.png",
    "default_popup": "popup.html"
  }
}

Go ahead and save that data to a file named manifest.json in the directory you created, or download a copy of manifest.json from our sample repository .

What does it mean?

The attribute names are fairly self-descriptive, but let's walk through the manifest line-by-line to make sure we're all on the same page.

The first line, which declares that we're using version 2 of the manifest file format, is mandatory (version 1 is old, deprecated, and generally not awesome).

The next block defines the extension's name, description, and version. These will be used both inside of Chrome to show a user which extensions you have installed, and also on the Chrome Web Store to display your extension to potentially new users. The name should be short and snappy, and the description no longer than a sentence or so (you'll have more room for a detailed description later).

The final block first requests permission to work with data on https://secure.flickr.com/, and declares that this extension implements a browser action, assigning it a default icon and popup in the process.

Resources

You probably noticed that manifest.json pointed at two resource files when defining the browser action: icon.png and popup.html. Both resources must exist inside the extension package, so let's create them now:

You should now have four files in your working directory: icon.png, manifest.json, popup.html, popup.js. The next step is to load those files into Chrome.

Load the extension

Extensions that you download from the Chrome Web Store are packaged up as .crx files, which is great for distribution, but not so great for development. Recognizing this, Chrome gives you a quick way of loading up your working directory for testing. Let's do that now.

  1. Visit chrome://extensions in your browser (or open up the Chrome menu by clicking the icon to the far right of the Omnibox: The menu's icon is three horizontal bars.. and select Extensions under the Tools menu to get to the same place).

  2. Ensure that the Developer mode checkbox in the top right-hand corner is checked.

  3. Click Load unpacked extension… to pop up a file-selection dialog.

  4. Navigate to the directory in which your extension files live, and select it.

If the extension is valid, it'll be loaded up and active right away! If it's invalid, an error message will be displayed at the top of the page. Correct the error, and try again.

Fiddle with Code

Now that you've got your first extension up and running, let's fiddle with things so that you have an idea what your development process might look like. As a trivial example, let's change the data source to search for pictures of puppies instead of kittens.

Hop into popup.js, and edit line 11 from var QUERY = 'kittens'; to read var QUERY = 'puppies';, and save your changes.

If you click on your extension's browser action again, you'll note that your change hasn't yet had an effect. You'll need to let Chrome know that something has happened, either explicitly by going back to the extension page (chrome://extensions, or Tools > Extensions under the Chrome menu), and clicking Reload under your extension, or by reloading the extensions page itself (either via the reload button to the left of the Omnibox, or by hitting F5 or Ctrl-R).

Once you've reloaded the extension, click the browser action icon again. Puppies galore!

What next?

You now know about the manifest file's central role in bringing things together, and you've mastered the basics of declaring a browser action, and rendering some kittens (or puppies!) in response to a user's click. That's a great start, and has hopefully gotten you interested enough to explore further. There's a lot more out there to play around with.