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.

Summary of extension types showing browser extensions, packaged/platform apps,and hosted/bookmark apps

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 combined the appearance of a 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 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.