extensions/docs/extension_and_app_types.md - chromium/src - Git at Google

 # Extension and App Types

 Generally, browser extensions cut across websites and web apps, while apps
 provide more isolated functionality. Read on for specifics.

 **This is a technical discussion of extension types for Chromium developers.**
 Extension developers should refer to http://developer.chrome.com/ for
 documentation, usage guidelines and examples.

 [TOC]

 ![Summary of extension types showing browser extensions, packaged/platform apps,
 and hosted/bookmark apps](extension_types.png)

 ## Browser extensions

 Browser extensions often provide an interactive toolbar icon, but can also run
 without any UI. They may interact with the browser or tab contents, and can
 request more extensive permissions than apps.

 A browser extension can be identified by a `manifest.json` file without any key
 named `app`, `export`, or `theme`.

 ## Themes

 A theme is a special kind of extension that changes the way the browser looks.
 Themes are packaged like regular extensions, but they don't contain JavaScript
 or HTML code.

 A theme can be identified by the presence of a `theme` key in `manifest.json`.

 ## Shared Modules

 Shared modules are permissionless collections of resources that can be shared
 between other extensions and apps.

 A shared module can be identified by the presence of an `export` key in
 `manifest.json`.

 ## Apps

 ### Platform app

 Platform apps (*v2 packaged apps*) are standalone applications that mostly run
 independently of the browser. Their windows look and feel like native
 applications but simply host the app's pages.

 Most apps, like Calculator and the Files app, create their window(s) and
 initialize a UI in response to Chrome's `chrome.app.runtime.onLaunched` event.
 Some apps don't show a window but work in the background instead. Platform apps
 can connect to more device types than browser extensions have access to.

 A platform app can be identified by the presence of an `app.background` key
 in the manifest, which provides the script that runs when the app is
 launched.

 *Platform apps are deprecated on non-Chrome OS platforms.*

 ### Packaged app (legacy)

 [Legacy (v1) packaged apps](https://developer.chrome.com/extensions/apps)
 combined the appearance of a [hosted app](#Hosted-app) -- a windowed wrapper
 around a website -- with the power of extension APIs. With the launch of
 platform apps and the app-specific APIs, legacy packaged apps are deprecated.

 A packaged app can be identified by the presence of an
 `app.launch.local_path` key in `manifest.json`, which identifies the resource
 in the .crx that's loaded when the app is launched.

 *Packaged apps are deprecated everywhere.*

 ### Hosted app

 A [hosted app](https://developer.chrome.com/webstore/hosted_apps) is mostly
 metadata: a web URL to launch, a list of associated URLs, and a list of
 permissions. Chrome asks for these permissions during the app's installation.
 Some permissions allow the associated URL to bypass runtime permission prompts
 of regular web features. Other than metadata in the manifest and icons, none of
 a hosted app's resources come from the extension system.

 A hosted app can declare a BackgroundContents, which outlives the browser and
 can be scripted from all tabs running the hosted app. Specifying
 `allow_js_access: false` is preferred, to allow multiple instances of the hosted
 app to run in different processes.

 A hosted app can be identified by the presence of an `app.launch.web_url` key in
 `manifest.json`, which provides http/https URL that is loaded when the app is
 launched.

 *Hosted apps are deprecated on non-Chrome OS platforms.*

 ### Bookmark app

 A bookmark app is a simplified hosted app that Chrome creates on demand. When
 the user taps "More Tools > Add to desktop..." (or "Add to shelf" on Chrome OS)
 in the Chrome menu, Chrome creates a barebones app whose manifest specifies the
 current tab's URL. A shortcut to this URL appears in chrome://apps using the
 site's favicon.

 Chrome then creates a desktop shortcut that will open a browser window with
 flags that specify the app and profile. Activating the icon launches the
 "bookmarked" URL in a tab or a window.

 A bookmark app's `manifest.json` identifies it as a hosted app. However, in the
 C++ code, the `Extension` object will return true from its `from_bookmark()`
 method.

 ### Progressive Web App (PWA)

 When Progressive Web Apps are installed on desktop a bookmark app is created.
 The bookmark app in this case will capture navigations to its scope and opens
 them in a dedicated app window instead of the existing browser context.

 ## Ambiguity surrounding the term "Extension"

 In the C++ code, all of the above flavors of extensions and apps are implemented
 in terms of the `Extension` class type, and the `//extensions` module. This can
 cause confusion, since it means that an app *is-an* `Extension`, although
 `Extension::is_extension()` is false.

 In code comments, "extension" may be used to refer to non-app extensions, also
 known as *browser extensions*.

 The three categories of apps are significantly different in terms of
 implementation, capabilities, process model, and restrictions. It is usually
 necessary to consider them as three separate cases, rather than lumping them
 together.

 ## See also

 * [`Manifest::Type` declaration](https://cs.chromium.org/chromium/src/extensions/common/manifest.h?gs=cpp%253Aextensions%253A%253Aclass-Manifest%253A%253Aenum-Type%2540chromium%252F..%252F..%252Fextensions%252Fcommon%252Fmanifest.h%257Cdef&gsn=Type&ct=xref_usages)
 * Extensions (3rd-party developer documentation)
     * [Extension APIs](https://developer.chrome.com/extensions/api_index)
     * [Extension manifest file format](
       https://developer.chrome.com/extensions/manifest)
 * Apps (3rd-party developer documentation)
     * [Platform app APIs](https://developer.chrome.com/apps/api_index)
     * [Platform app manifest file format](
       https://developer.chrome.com/apps/manifest)
     * [Choosing an app type](https://developer.chrome.com/webstore/choosing)
     * Ancient article introducing the [motivation for apps (outdated)](
       https://developer.chrome.com/webstore/apps_vs_extensions)
	# Extension and App Types

	Generally, browser extensions cut across websites and web apps, while apps
	provide more isolated functionality. Read on for specifics.

	This is a technical discussion of extension types for Chromium developers.
	Extension developers should refer to http://developer.chrome.com/ for
	documentation, usage guidelines and examples.

	[TOC]

	![Summary of extension types showing browser extensions, packaged/platform apps,
	and hosted/bookmark apps](extension_types.png)

	## Browser extensions

	Browser extensions often provide an interactive toolbar icon, but can also run
	without any UI. They may interact with the browser or tab contents, and can
	request more extensive permissions than apps.

	A browser extension can be identified by a `manifest.json` file without any key
	named `app`, `export`, or `theme`.

	## Themes

	A theme is a special kind of extension that changes the way the browser looks.
	Themes are packaged like regular extensions, but they don't contain JavaScript
	or HTML code.

	A theme can be identified by the presence of a `theme` key in `manifest.json`.

	## Shared Modules

	Shared modules are permissionless collections of resources that can be shared
	between other extensions and apps.

	A shared module can be identified by the presence of an `export` key in
	`manifest.json`.

	## Apps

	### Platform app

	Platform apps (v2 packaged apps) are standalone applications that mostly run
	independently of the browser. Their windows look and feel like native
	applications but simply host the app's pages.

	Most apps, like Calculator and the Files app, create their window(s) and
	initialize a UI in response to Chrome's `chrome.app.runtime.onLaunched` event.
	Some apps don't show a window but work in the background instead. Platform apps
	can connect to more device types than browser extensions have access to.

	A platform app can be identified by the presence of an `app.background` key
	in the manifest, which provides the script that runs when the app is
	launched.

	Platform apps are deprecated on non-Chrome OS platforms.

	### Packaged app (legacy)

	[Legacy (v1) packaged apps](https://developer.chrome.com/extensions/apps)
	combined the appearance of a [hosted app](#Hosted-app) -- a windowed wrapper
	around a website -- with the power of extension APIs. With the launch of
	platform apps and the app-specific APIs, legacy packaged apps are deprecated.

	A packaged app can be identified by the presence of an
	`app.launch.local_path` key in `manifest.json`, which identifies the resource
	in the .crx that's loaded when the app is launched.

	Packaged apps are deprecated everywhere.

	### Hosted app

	A [hosted app](https://developer.chrome.com/webstore/hosted_apps) is mostly
	metadata: a web URL to launch, a list of associated URLs, and a list of
	permissions. Chrome asks for these permissions during the app's installation.
	Some permissions allow the associated URL to bypass runtime permission prompts
	of regular web features. Other than metadata in the manifest and icons, none of
	a hosted app's resources come from the extension system.

	A hosted app can declare a BackgroundContents, which outlives the browser and
	can be scripted from all tabs running the hosted app. Specifying
	`allow_js_access: false` is preferred, to allow multiple instances of the hosted
	app to run in different processes.

	A hosted app can be identified by the presence of an `app.launch.web_url` key in
	`manifest.json`, which provides http/https URL that is loaded when the app is
	launched.

	Hosted apps are deprecated on non-Chrome OS platforms.

	### Bookmark app

	A bookmark app is a simplified hosted app that Chrome creates on demand. When
	the user taps "More Tools > Add to desktop..." (or "Add to shelf" on Chrome OS)
	in the Chrome menu, Chrome creates a barebones app whose manifest specifies the
	current tab's URL. A shortcut to this URL appears in chrome://apps using the
	site's favicon.

	Chrome then creates a desktop shortcut that will open a browser window with
	flags that specify the app and profile. Activating the icon launches the
	"bookmarked" URL in a tab or a window.

	A bookmark app's `manifest.json` identifies it as a hosted app. However, in the
	C++ code, the `Extension` object will return true from its `from_bookmark()`
	method.

	### Progressive Web App (PWA)

	When Progressive Web Apps are installed on desktop a bookmark app is created.
	The bookmark app in this case will capture navigations to its scope and opens
	them in a dedicated app window instead of the existing browser context.

	## Ambiguity surrounding the term "Extension"

	In the C++ code, all of the above flavors of extensions and apps are implemented
	in terms of the `Extension` class type, and the `//extensions` module. This can
	cause confusion, since it means that an app is-an `Extension`, although
	`Extension::is_extension()` is false.

	In code comments, "extension" may be used to refer to non-app extensions, also
	known as browser extensions.

	The three categories of apps are significantly different in terms of
	implementation, capabilities, process model, and restrictions. It is usually
	necessary to consider them as three separate cases, rather than lumping them
	together.

	## See also

	* [`Manifest::Type` declaration](https://cs.chromium.org/chromium/src/extensions/common/manifest.h?gs=cpp%253Aextensions%253A%253Aclass-Manifest%253A%253Aenum-Type%2540chromium%252F..%252F..%252Fextensions%252Fcommon%252Fmanifest.h%257Cdef&gsn=Type&ct=xref_usages)
	* Extensions (3rd-party developer documentation)
	* [Extension APIs](https://developer.chrome.com/extensions/api_index)
	* [Extension manifest file format](
	https://developer.chrome.com/extensions/manifest)
	* Apps (3rd-party developer documentation)
	* [Platform app APIs](https://developer.chrome.com/apps/api_index)
	* [Platform app manifest file format](
	https://developer.chrome.com/apps/manifest)
	* [Choosing an app type](https://developer.chrome.com/webstore/choosing)
	* Ancient article introducing the [motivation for apps (outdated)](
	https://developer.chrome.com/webstore/apps_vs_extensions)