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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 fetch an
  image from Google using the current page's URL as a search term.
</p>

<p>
  We'll do so by implementing a UI element we call a
  <a href="browserAction">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 an image derived from the current
  page, which will look something like this:
</p>

<img src="{{static}}/images/gettingstarted-preview"
     width="558"
     height="264"
     alt="Chrome with an extension's popup open.">

<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>. This manifest is nothing more than a metadata file
  in JSON format that contains properties like your extension's name,
  description, version number and so on. At a high level, we will use it to
  declare to Chrome what the extension is going to do, and what permissions it
  requires in order to do those things. To learn more about the manifest, read
  the <a href="manifest">Manifest File Format documentation</a>.
</p>

<p>
  In our example's manifest,
  we will declare a <a href="browserAction">browser action</a>,
  the <a href="activeTab">activeTab permission</a> to see the URL of the current
  tab, and the host <a href="declare_permissions">permission</a> to access the
  external Google Image search API.
</p>

<pre data-filename="manifest.json">
{
  "manifest_version": 2,

  "name": "Getting started example",
  "description": "This extension shows a Google Image search result for the current page",
  "version": "1.0",

  "browser_action": {
    "default_icon": "icon.png",
    "default_popup": "popup.html"
  },
  "permissions": [
    "activeTab",
    "https://ajax.googleapis.com/"
  ]
}
</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>

<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"
           style="float:right"
           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.
      <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>
      <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>
      The actual logic of rendering the content of the popup is implemented by
      <a href="examples/tutorials/getstarted/popup.js">popup.js</a>. You are
      encouraged to read the elaborate comments in this file to learn more
      about the logic.<br>
      <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>
  Alternatively, you can drag and drop the directory where your extension files
  live onto <code>chrome://extensions</code> in your browser to load it.
</p>

<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.
  For example, let's set a tooltip on the browser action button.
</p>
<p>
  According to the browserAction documentation, tooltips can be set by
  specifying the <code>default_title</code> key in the manifest file. Open
  <code>manifest.json</code>, and add the <code>default_title</code> key to the
  <code>browser_action</code>.
  Make sure that the JSON is valid, so quote the key and add a comma where necessary.
</p>

<pre data-filename="manifest.json">
{
  ...
  "browser_action": {
    "default_icon": "icon.png",
    "default_popup": "popup.html",
    "default_title": "Click here!"
  },
  ...
}
</pre>

<p>
  The manifest file is only parsed when the extension is loaded. If you want to
  see the previous changes in action, the extension has to be reloaded.
  Visit the extensions page (go to <strong>chrome://extensions</strong>, or
  <strong>Tools &gt; Extensions</strong> under the Chrome menu), and click
  <strong>Reload</strong> under your extension.
  All extensions are also reloaded when the extensions page is reloaded, e.g.
  after hitting <kbd>F5<kbd> or <kbd>Ctrl</kbd>-<kbd>R</kbd>.
</p>

<p>
  Once you've reloaded the extension, hover over the browser action badge to see
  the new tooltip!<br>
  <img src="{{static}}/images/gettingstarted-tooltip-before.png"
       width="160"
       height="120"
       alt="&quot;Getting started example&quot; tooltip.">

  <img src="{{static}}/images/gettingstarted-tooltip-after.png"
       width="160"
       height="120"
       alt="&quot;Click here!&quot; tooltip, after modifying manifest.json and reloading the extension.">
</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.
  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">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">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
      iceberg. Our <a href="api_index">chrome.* APIs documentation</a> will
      walk you through each API in turn.
    </p>
  </li>
  <li>
    <p>
      Finally, the <a href="devguide">developer's guide</a> has dozens of
      additional links to pieces of documentation you might be interested in.
    </p>
  </li>
</ul>