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<h1>Getting Started: Building a Chrome Extension</h1>

<p>
  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!
</p>

<p>
  We'll do so by implementing a UI element we call a
  <a href="browserAction.html">browser action</a>, 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:
</p>

<img src="{{static}}/images/gettingstarted-1.jpg"
     width="600"
     height="420"
     alt="Chrome, with an extension's popup open and displaying many kittens.">

<p>
  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!
</p>

<h2 id="declaration">Something to Declare</h2>

<p>
  The very first thing we'll need to create is a <dfn>manifest file</dfn> named
  <code>manifest.json</code>. 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.
</p>

<p>
  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:
</p>

<pre data-filename="manifest.json">
{
  "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"
  }
}
</pre>

<p>
  Go ahead and save that data to a file named <code>manifest.json</code> in the
  directory you created, or
  <a href="examples/tutorials/getstarted/manifest.json" download="manifest.json">
    download a copy of <code>manifest.json</code> from our sample repository
  </a>.
</p>

<h3 id="manifest">What does it mean?</h3>

<p>
  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.
</p>

<p>
  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).
</p>

<p>
  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).
</p>

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

<h2 id="resources">Resources</h2>

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

<ul class="imaged">
  <li>
    <p>
      <img src="{{static}}/images/gettingstarted-icon.png"
           width="127"
           height="127"
           alt="The popup's icon will be displayed right next to the Omnibox.">
      <code>icon.png</code> will be displayed next to the Omnibox, waiting for
      user interaction. Download a copy of icon.png from our sample repository,
      <a href="examples/tutorials/getstarted/icon.png" download="icon.png">
        Download a copy of <code>icon.png</code> from our sample repository
      </a>, and save it into the directory you're working in. You could also
      create your own if you're so inclined; it's just a 19px-square PNG file.
    </p>
  </li>
  <li>
    <p>
      <img src="{{static}}/images/gettingstarted-popup.jpg"
           width="165"
           height="200"
           alt="The popup's HTML will be rendered directly below the icon when clicked.">
      <code>popup.html</code> will be rendered inside the popup window that's
      created in response to a user's click on the browser action. It's a
      standard HTML file, just like you're used to from web development, giving
      you more or less free reign over what the popup displays.
      <a href="examples/tutorials/getstarted/popup.html" download="popup.html">
        Download a copy of <code>popup.html</code> from our sample repository
      </a>, and save it into
      the directory you're working in.
    </p>
    <p>
      <code>popup.html</code> requires an additional JavaScript file in order to
      do the work of grabbing kitten images from the web and loading them into
      the popup. To save you some effort, just
      <a href="examples/tutorials/getstarted/popup.js" download="popup.js">
        download a copy of <code>popup.js</code> from our sample repository
      </a>, and save it into the directory you're working in.
    </p>
  </li>
</ul>

<p>
  You should now have four files in your working directory:
  <a href="examples/tutorials/getstarted/icon.png" download="icon.png"><code>icon.png</code></a>,
  <a href="examples/tutorials/getstarted/manifest.json" download="manifest.json"><code>manifest.json</code></a>,
  <a href="examples/tutorials/getstarted/popup.html" download="popup.html"><code>popup.html</code></a>,
  <a href="examples/tutorials/getstarted/popup.js" download="popup.js"><code>popup.js</code></a>.
  The next step is to load those files into Chrome.
</p>

<h2 id="unpacked">Load the extension</h2>

<p>
  Extensions that you download from the Chrome Web Store are packaged up as
  <code>.crx</code> 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.
</p>

<ol>
  <li>
    <p>
      Visit <code>chrome://extensions</code> in your browser (or open up the
      Chrome menu by clicking the icon to the far right of the Omnibox:
      <img src="{{static}}/images/hotdogmenu.png"
           height="29"
           width="29"
           alt="The menu's icon is three horizontal bars.">. and
      select <strong>Extensions</strong> under the <strong>Tools</strong> menu
      to get to the same place).
    </p>
  </li>
  <li>
    <p>
      Ensure that the <strong>Developer mode</strong> checkbox in the top
      right-hand corner is checked.
    </p>
  </li>
  <li>
    <p>
      Click <strong>Load unpacked extension&hellip;</strong> to pop up a
      file-selection dialog.
    </p>
  </li>
  <li>
    <p>
      Navigate to the directory in which your extension files live, and select
      it.
    </p>
  </li>
</ol>

<p>
  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.
</p>

<h2 id="update-code">Fiddle with Code</h2>

<p>
  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.
</p>

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

<p>
  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
  (<strong>chrome://extensions</strong>, or
  <strong>Tools &gt; Extensions</strong> under the Chrome menu), and clicking
  <strong>Reload</strong> 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).
</p>

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

<h2 id="next-steps">What next?</h2>

<p>
  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.
</p>

<ul>
  <li>
    <p>
      The <a href="overview.html">Chrome Extension Overview</a> backs up a bit,
      and fills in a lot of detail about extensions' architecture in general,
      and some specific concepts you'll want to be familiar with going forward.
      It's the best next step on your journey towards extension mastery.
    </p>
  </li>
  <li>
    <p>
      No one writes perfect code on the first try, which means that you'll need
      to learn about the options available for debugging your creations. Our
      <a href="tut_debugging.html">debugging tutorial</a> is perfect for that,
      and is well worth carefully reading.
    </p>
  </li>
  <li>
    <p>
      Chrome extensions have access to powerful APIs above and beyond what's
      available on the open web: browser actions are just the tip of the
      iceburg. Our <a href="api_index.html">chrome.* APIs documentation</a> will
      walk you through each API in turn.
    </p>
  </li>
  <li>
    <p>
      Finally, the <a href="devguide.html">developer's guide</a> has dozens of
      additional links to pieces of documentation you might be interested in.
    </p>
  </li>
</ul>