<div id="pageData-title" class="pageData">Overview</div>
<div id="pageData-showTOC" class="pageData">true</div>

<p>
Once you've finished this page
and the
<a href="getstarted.html">Getting Started</a> tutorial,
you'll be all set to start writing extensions.
</p>


<h2 id="what">The basics</h2>

<p>
An extension is a zipped bundle of files &mdash;
HTML, CSS, JavaScript, images, and anything else you need &mdash;
that adds functionality to the Google Chrome browser.
Extensions are essentially web pages,
and they can use all the APIs
that the browser provides
to web pages,
from XMLHttpRequest to JSON to HTML5 local storage.
</p>

<p>
Many extensions add UI to Google Chrome,
in the form of
<a href="browserAction.html">browser actions</a>
or <a href="pageAction.html">page actions</a>.
Extensions can also interact programmatically
with browser features such as
<a href="bookmarks.html">bookmarks</a>
and <a href="tabs.html">tabs</a>.
To interact with web pages or servers,
extensions can use
<a href="content_scripts.html">content scripts</a> or
<a href="xhr.html">cross-origin XMLHttpRequests</a>.
</p>

<p>
<b>Note:</b>
Each extension can have at most one browser action or page action.
Choose a <b>browser action</b> when the extension is relevant to most pages.
Choose a <b>page action</b> when the extension's icon
should appear or disappear,
depending on the page.
</p>

<p>
See the <a href="devguide.html">Developer's Guide</a>
for a complete list of extension features,
with implementation details
for each one.
</p>

<h2 id="files">Files</h2>
<p>
Each extension has the following files:
<!-- PENDING: This could use a picture -->
</p>

<ul>
  <li>A <b>manifest file</b></li>
  <li>One or more <b>HTML files</b> (unless the extension is a theme)</li>
  <li><em>Optional:</em> One or more <b>JavaScript files</b></li>
  <li><em>Optional:</em> Any other files your extension needs &mdash;
  for example, image files</li>
</ul>

<p>
While you're working on your extension,
you put all these files into a single folder.
When you distribute your extension,
the contents of the folder are packaged into a special ZIP file
that has a <code>.crx</code> suffix,
as described in <a href="packaging.html">Packaging</a>.
</p>


<h3>Referring to files</h3>

<p>
You can put any file you like into an extension,
but how do you use it?
Usually,
you can refer to the file using a relative URL,
just as you would in an ordinary HTML page.
Here's an example of referring to
a file named <code>myimage.png</code>
that's in a subfolder named <code>images</code>.
</p>

<pre>
&lt;img <b>src="images/myimage.png"</b>>
</pre>

<p>
As you might notice while you use the Google Chrome debugger,
every file in an extension is also accessible by an absolute URL like this:
</p>

<blockquote>
<b>chrome-extension://</b><em>&lt;extensionID></em><b>/</b><em>&lt;pathToFile></em>
</blockquote>

<p>
In that URL, the <em>&lt;extensionID></em> is a unique identifier
that the extension system generates for each extension.
You can see the IDs for all your loaded extensions
by going to the URL <b>chrome://extensions</b>.
The <em>&lt;pathToFile></em> is the location of the file
under the extension's top folder;
it's the same as the relative URL.
</p>

<!-- [PENDING: Should mention/reflect/link to <a href="http://dev.chromium.org/developers/design-documents/extensions/i18n">internationalization</a> when it's ready.] -->


<h3>The manifest file</h3>

<p>
The manifest file, called <code>manifest.json</code>,
gives information about the extension,
such as the most important files
and the capabilities that the extension might use.
Here's a typical manifest file for a browser action
that uses information from google.com:
</p>

<pre>
{
  "name": "My Extension",
  "version": "2.1",
  "description": "Gets information from Google.",
  "icons": { "128": "icon_128.png" },
  "background_page": "bg.html",
  "permissions": ["http://*.google.com/", "https://*.google.com/"],
  "browser_action": {
    "default_title": "",
    "default_icon": "icon_19.png",
    "popup": "popup.html"
  }
}</pre>

<p>
For details, see
<a href="manifest.html">Manifest Files</a>.
</p>

<h2 id="arch">Architecture</h2>

<p>
Most extensions have a
<a href="background_pages.html">background page</a>,
an invisible page
that holds the main logic of the extension.
</p>

<img src="images/arch-1.gif">

<p>
The preceding figure shows a browser
that has at least two extensions installed:
a browser action (yellow icon)
and a page action (blue icon).
The background page,
which is defined by an HTML file
(<code>background.html</code>),
has JavaScript code that controls both
the browser action and the page action.
</p>


<h3 id="pages">Pages</h3>

<p>
The background page isn't the only HTML page
that an extension can have.
For example, a browser action can have a popup,
which is implemented by an HTML file.
Extensions can also
use <a href="tabs.html#Method-create"><code>chrome.tabs.create()</code></a>
or <code>window.open()</code>
to display HTML files that are in the extension.
</p>

<p>
The HTML pages inside an extension
have complete access to each other's DOMs,
and they can invoke functions on each other.
</p>

<p>
The following figure shows the architecture
of a browser action's popup.
The popup's contents are a web page
defined by an HTML file
(<code>popup.html</code>).
The popup doesn't need to duplicate code
that's in the background page
(<code>background.html</code>)
because the popup can invoke functions on the background page.
</p>

<img src="images/arch-2.gif">

<p>
See the <a href="browserAction.html">Browser Actions</a> page and
the <a href="#pageComm">Communication between pages</a> section
for more details.
</p>


<h3 id="contentScripts">Content scripts</h3>

<p>
If your extension needs to interact with web pages,
then it needs a <em>content script</em>.
A content script is some JavaScript
that executes in the context of a page
that's been loaded into the browser.
Think of a content script as part of that loaded page,
not as part of the extension it was packaged with
(its <em>parent extension</em>).
</p>

<!-- [PENDING: Consider explaining that the reason content scripts are separated from the extension is due to chrome's multiprocess design.  Something like:

Each extension runs in its own process.
To have rich interaction with a web page, however,
the extension must be able to
run some code in the web page's process.
Extensions accomplish this with content scripts.]
-->

<p>
Content scripts can read details of the web pages the browser visits,
and they can make changes to the pages.
In the following figure,
the content script
can read and modify
the DOM for the displayed web page.
It cannot, however, modify the DOM of its parent extension's background page.
</p>

<img src="images/arch-3.gif">

<p>
Content scripts aren't completely cut off from their parent extensions.
A content script can exchange messages with its parent extension,
as the arrows in the following figure show.
For example, a content script might send a message
whenever it finds an RSS feed in a browser page.
Or a background page might send a message
asking a content script to change the appearance of its browser page.
</p>

<img src="images/arch-cs.gif">

<!-- [PENDING: Add overview of message passing.] -->

<p>
For more information,
see <a href="content_scripts.html">Content Scripts</a>.
</p>


<h2 id="pageComm">Communication between pages </h2>

<p>
The HTML pages within an extension often need to communicate.
<!-- [PENDING: For example, ...] -->
Because all of an extension's pages
execute in same process on the same thread,
the pages can make direct function calls to each other.
</p>

<p>
To find pages in the extension, use
<a href="extension.html"><code>chrome.extension</code></a>
methods such as
<code>getViews()</code> and
<code>getBackgroundPage()</code>.
Once a page has a reference to other pages within the extension,
the first page can invoke functions on the other pages,
and it can manipulate their DOMs.
</p>

<!-- [PENDING: Here's an example of communication between xyz and the background page.  (code example goes here)] -->


<h2> Now what? </h2>

<p>
Now that you've been introduced to extensions,
you should be ready to write your own.
Here are some ideas for where to go next:
</p>

<ul>
  <li> <a href="getstarted.html">Tutorial: Getting Started</a> </li>
  <li> <a href="tut_debugging.html">Tutorial: Debugging</a> </li>
  <li> <a href="devguide.html">Developer's Guide</a> </li>
  <li> <a href="http://dev.chromium.org/developers/design-documents/extensions/samples">Samples</a> </li>
</ul>