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<h1>Overview</h1>


<p>
Once you've finished this page
and the
<a href="getstarted">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
<a href="api_other">APIs that the browser provides to web pages</a>,
from XMLHttpRequest to JSON to HTML5.
</p>

<p>
Extensions can interact with web pages or servers using
<a href="content_scripts">content scripts</a> or
<a href="xhr">cross-origin XMLHttpRequests</a>.
Extensions can also interact programmatically
with browser features such as
<a href="bookmarks">bookmarks</a>
and <a href="tabs">tabs</a>.
</p>

<h3 id="extension-ui">Extension UIs</h3>

<p>
Many extensions&mdash;but not Chrome Apps&mdash;add
UI to Google Chrome in the form of
<a href="browserAction">browser actions</a>
or <a href="pageAction">page actions</a>.
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>

<table class="simple">
<tr>
  <td width="33%">
    <img src="{{static}}/images/overview/browser-action.png"
      width="147" height="100"
      alt="screenshot" />
  </td>
  <td width="33%">
    <img src="{{static}}/images/overview/page-action.png"
      width="147" height="100"
      alt="screenshot" />
  </td>
  <td>
    <img src="{{static}}/images/overview/browser-action-with-popup.png"
      width="147" height="100"
      alt="screenshot" />
  </td>
</tr>

<tr>
  <td>
    This <a href="samples#google-mail-checker">Google Mail Checker extension</a>
    uses a <em>browser action</em>
    (icon in the toolbar).
  </td>
  <td>
    This <a href="samples#mappy">Mappy extension</a>
    uses a <em>page action</em>
    (icon in the address bar)
    and <em>content script</em>
    (code injected into a web page).
  </td>
  <td>
    This <a href="samples#news-reader">News Reader extension</a>
    features a browser action that,
    when clicked,
    shows a <em>popup</em>.
  </td>
</tr>
</table>

<p>
Extensions (and Chrome Apps) can also present a UI in other ways,
such as adding to the Chrome context menu,
providing an options page,
or using a content script that changes how pages look.
See the <a href="devguide">Developer's Guide</a>
for a complete list of extension features,
with links to implementation details
for each one.
</p>

<h2 id="files">Files</h2>
<p>
Each extension has the following files:

</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.
If you upload your extension using the
<a href="https://chrome.google.com/webstore/developer/dashboard">Chrome Developer Dashboard</a>,
the <code>.crx</code> file is created for you.
For details on distributing extensions,
see <a href="hosting">Hosting</a>.
</p>


<h3 id="relative-urls">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>

<p>
While you're working on an extension
(before it's packaged),
the extension ID can change.
Specifically, the ID of an unpacked extension will change
if you load the extension from a different directory;
the ID will change again when you package the extension.
If your extension's code
needs to specify the full path to a file within the extension,
you can use the <code>@@extension_id</code>
<a href="i18n#overview-predefined">predefined message</a>
to avoid hardcoding the ID during development.
</p>

<p>
When you package an extension
(typically, by uploading it with the dashboard),
the extension gets a permanent ID,
which remains the same even after you update the extension.
Once the extension ID is permanent,
you can change all occurrences of
<code>@@extension_id</code> to use the real ID.
</p>


<h3 id="manifest">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 data-filename="manifest.json">
{
  "name": "My Extension",
  "version": "2.1",
  "description": "Gets information from Google.",
  "icons": { "128": "icon_128.png" },
  "background": {
    "persistent": false,
    "scripts": ["bg.js"]
  },
  "permissions": ["http://*.google.com/", "https://*.google.com/"],
  "browser_action": {
    "default_title": "",
    "default_icon": "icon_19.png",
    "default_popup": "popup.html"
  }
}</pre>

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

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

<p>
Many extensions have a <em>background page</em>,
an invisible page
that holds the main logic of the extension.
An extension can also contain other pages
that present the extension's UI.
If an extension needs to interact with web pages that the user loads
(as opposed to pages that are included in the extension),
then the extension must use a content script.
</p>


<h3 id="background_page">The background page</h3>

<p>
The following figure shows a browser
that has at least two extensions installed:
a browser action (yellow icon)
and a page action (blue icon).
Both the browser action and the page action
have background pages.
This figure shows the browser action's background page,
which is defined by <code>background.html</code>
and has JavaScript code that controls
the behavior of the browser action in both windows.
</p>

<img src="{{static}}/images/overview/arch-1.gif"
 width="232" height="168"
 alt="Two windows and a box representing a background page (background.html). One window has a yellow icon; the other has both a yellow icon and a blue icon. The yellow icons are connected to the background page." />

<p>
There are two types of background pages:
<a href="background_pages">persistent background pages</a>,
and <a href="event_pages">event pages</a>. Persistent
background pages, as the name suggests, are always open.
Event pages are opened and closed as needed. Unless you absolutely
need your background page to run all the time, prefer to use
an event page.
</p>

<!-- PENDING: Perhaps show a picture of many background page processes.
  This could build on a figure that shows the process architecture. -->

<p>
See <a href="event_pages">Event Pages</a>
and <a href="background_pages">Background Pages</a>
for more details.
</p>

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

<p>
Extensions can contain ordinary HTML pages that display the extension's UI.
For example, a browser action can have a popup,
which is implemented by an HTML file.
Any extension can have an options page,
which lets users customize how the extension works.
Another type of special page is the override page.
And finally, you can
use $(ref:tabs.create)
or <code>window.open()</code>
to display any other 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>

<!-- PENDING: Change the following example and figure
to use something that's not a popup?
(It might lead people to think that popups need background pages.) -->

<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>).
This extension also happens to have a background page
(<code>background.html</code>).
The popup doesn't need to duplicate code
that's in the background page
because the popup can invoke functions on the background page.
</p>

<img src="{{static}}/images/overview/arch-2.gif"
 width="256" height="168"
 alt="A browser window containing a browser action that's displaying a popup. The popup's HTML file (popup.html) can communicate with the extension's background page (background.html)." />

<p>
See <a href="browserAction">Browser Actions</a>,
<a href="options">Options</a>,
<a href="override">Override Pages</a>,
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="{{static}}/images/overview/arch-3.gif"
 width="238" height="169"
 alt="A browser window with a browser action (controlled by background.html) and a content script (controlled by contentscript.js)." />

<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="{{static}}/images/overview/arch-cs.gif"
 width="238" height="194"
 alt="Like the previous figure, but showing more of the parent extension's files, as well as a communication path between the content script and the parent extension." />



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


<h2 id="apis"> Using the chrome.* APIs </h2>

<p>
In addition to having access to all the APIs that web pages and apps can use,
extensions can also use Chrome-only APIs
(often called <em>chrome.* APIs</em>)
that allow tight integration with the browser.
For example, any extension or web app can use the
standard <code>window.open()</code> method to open a URL.
But if you want to specify which window that URL should be displayed in,
your extension can use the Chrome-only
$(ref:tabs.create)
method instead.
</p>

<h3 id="sync"> Asynchronous vs. synchronous methods </h3>
<p>
Most methods in the chrome.* APIs are <b>asynchronous</b>:
they return immediately, without waiting for the operation to finish.
If you need to know the outcome of that operation,
then you pass a callback function into the method.
That callback is executed later (potentially <em>much</em> later),
sometime after the method returns.
Here's an example of the signature for an asynchronous method:
</p>

<p>
<code>
chrome.tabs.create(object <em>createProperties</em>, function <em>callback</em>)
</code>
</p>

<p>
Other chrome.* methods are <b>synchronous</b>.
Synchronous methods never have a callback
because they don't return until they've completed all their work.
Often, synchronous methods have a return type.
Consider the
$(ref:runtime.getURL) method:
</p>

<p>
<code>
string chrome.runtime.getURL()
</code>
</p>

<p>
This method has no callback and a return type of <code>string</code>
because it synchronously returns the URL
and performs no other, asynchronous work.
</p>


<h3 id="sync-example"> Example: Using a callback </h3>

<p>
Say you want to navigate
the user's currently selected tab to a new URL.
To do this, you need to get the current tab's ID
(using $(ref:tabs.query))
and then make that tab go to the new URL
(using $(ref:tabs.update)).
</p>

<p>
If <code>query()</code> were synchronous,
you might write code like this:
</p>

<pre>
<b>//THIS CODE DOESN'T WORK</b>
var tab = chrome.tabs.query({'active': true}); <b>//WRONG!!!</b>
chrome.tabs.update(tab.id, {url:newUrl});
someOtherFunction();
</pre>

<p>
That approach fails
because <code>query()</code> is asynchronous.
It returns without waiting for its work to complete,
and it doesn't even return a value
(although some asynchronous methods do).
You can tell that <code>query()</code> is asynchronous
by the <em>callback</em> parameter in its signature:

<p>
<code>
chrome.tabs.query(object <em>queryInfo</em>, function <em>callback</em>)
</code>
</p>

<p>
To fix the preceding code,
you must use that callback parameter.
The following code shows
how to define a callback function
that gets the results from <code>query()</code>
(as a parameter named <code>tab</code>)
and calls <code>update()</code>.
</p>

<pre>
<b>//THIS CODE WORKS</b>
chrome.tabs.query({'active': true}, <b>function(tabs) {</b>
  chrome.tabs.update(tabs[0].id, {url: newUrl});
<b>}</b>);
someOtherFunction();
</pre>

<p>
In this example, the lines are executed in the following order: 1, 4, 2.
The callback function specified to <code>query()</code> is called
(and line 2 executed)
only after information about the currently selected tab is available,
which is sometime after <code>query()</code> returns.
Although <code>update()</code> is asynchronous,
this example doesn't use its callback parameter,
since we don't do anything about the results of the update.
</p>


<h3 id="chrome-more"> More details </h3>

<p>
For more information, see the
<a href="api_index">chrome.* API docs</a>
and watch this video:
</p>

<div class="video-container">
  <iframe title="YouTube video player" width="640" height="390" src="//www.youtube.com/embed/bmxr75CV36A?rel=0" frameborder="0" allowfullscreen></iframe>
</div>

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

<p>
The HTML pages within an extension often need to communicate.

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"><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>

<h2 id="incognito"> Saving data and incognito mode </h2>

<p>
Extensions can save data using the $(ref:storage) API,
the HTML5 <a href="http://dev.w3.org/html5/webstorage/">web storage API</a>
(such as <code>localStorage</code>)
or by making server requests that result in saving data.
Whenever you want to save something,
first consider whether it's
from a window that's in incognito mode.
By default, extensions don't run in incognito windows.
You need to consider what a user expects
from your extension
when the browser is incognito.
</p>

<p>
<em>Incognito mode</em> promises that the window will leave no tracks.
When dealing with data from incognito windows,
do your best to honor this promise.
For example, if your extension normally
saves browsing history to the cloud,
don't save history from incognito windows.
On the other hand, you can store
your extension's settings from any window,
incognito or not.
</p>

<p class="note">
<b>Rule of thumb:</b>
If a piece of data might show where a user
has been on the web or what the user has done,
don't store it if it's from an incognito window.
</p>

<p>
To detect whether a window is in incognito mode,
check the <code>incognito</code> property of the relevant
$(ref:tabs.Tab) or
$(ref:windows.Window) object.
For example:
</p>

<pre>
function saveTabData(tab, data) {
  if (tab.incognito) {
    chrome.runtime.getBackgroundPage(function(bgPage) {
      bgPage[tab.url] = data;      // Persist data ONLY in memory
    });
  } else {
    localStorage[tab.url] = data;  // OK to store data
  }
}
</pre>


<h2 id="now-what"> 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">Tutorial: Getting Started</a> </li>
  <li> <a href="tut_debugging">Tutorial: Debugging</a> </li>
  <li> <a href="devguide">Developer's Guide</a> </li>
  <li> <a href="samples">Samples</a> </li>
  <li> <a href="http://www.youtube.com/view_play_list?p=CA101D6A85FE9D4B">Videos</a>,
    such as
    <a href="http://www.youtube.com/watch?v=B4M_a7xejYI&feature=PlayList&p=CA101D6A85FE9D4B&index=6">Extension Message Passing</a>
    </li>
</ul>