Which JavaScript engine is part of WebKit and used by Safari?

The JavaScript engine that is part of WebKit and used by Safari is JavaScriptCore.

JavaScriptCore began as Apple's fork of KJS, the JavaScript engine from KDE's Konqueror project. Apple adapted KJS when building Safari and WebKit, while the related KHTML code became WebCore, WebKit's layout and rendering component. Together, these components handle web pages and scripts in Apple's browser stack.

The name can cause confusion because Safari is often described simply as using WebKit. WebKit is the broader browser engine; JavaScriptCore is the scripting engine inside it. Likewise, V8 belongs chiefly to Chromium-based browsers and Node.js, while SpiderMonkey is associated with Firefox. Chakra was Microsoft's former engine for Internet Explorer and early Edge.

JavaScriptCore has also had project codenames, including SquirrelFish and Nitro, reflecting major interpreter and just-in-time compiler improvements. In modern WebKit, it remains responsible for executing JavaScript, separate from the engine's HTML, CSS, and rendering work.