Environments

A source runs in one of two environments. JavaScriptCore is the default. WebKit is optional, and you select it with the "use webkit" directive.

JavaScriptCore

JavaScriptCore is a JavaScript engine with no DOM, no window object and no page lifecycle.

Suwatte installs these objects before it evaluates your bundle:

• console, which writes to Settings → Advanced → Logs in the app.
• A store, which holds key and value pairs for your source only.
• The network stack. See Networking.
• Timers, such as setTimeout.
• A web view handle, to open a web view for a Cloudflare challenge.
• Crypto helpers and image helpers.

Each source gets its own context and its own serial queue. One source cannot see or change another source.

Use JavaScriptCore unless you have a reason not to. It starts more quickly, it uses less memory, and the rest of the interface is made for it.

WebKit

WebKit gives you a real web view. You get a DOM, a window object and all that a page context holds.

Use it when you cannot get the behaviour of the site without a browser. An example is a site that draws its pages with a large amount of client code.

WebKit has two costs:

• It uses much more memory, and it starts more slowly.
• useClientForImageRequests does not work. That option needs JavaScriptCore. The bootstrap throws an error if you declare it in a WebKit source.

The differences

	JavaScriptCore	WebKit
DOM	No	Yes
console to the app logs	Yes	Yes
Store	Yes	Yes
Network	Yes	Yes
useClientForImageRequests	Yes	No

Find the environment

The WebKit environment sets IS_WEBKIT_ENVIRONMENT. JavaScriptCore does not set it. Test for it with care:

const isWebKit =
  typeof IS_WEBKIT_ENVIRONMENT !== "undefined" && IS_WEBKIT_ENVIRONMENT;

Related

• Directives
• Networking