JavaScriptCore, renamed to SquirrelFish and marketed as
Nitro, for Safari
is a framework that provides a JavaScript engine for WebKit implementations, and provides this type of scripting in other contexts within OS X.
On June 2, 2008, the WebKit project announced they rewrote JavaScriptCore as "SquirrelFish", a bytecode interpreter. The project evolved into SquirrelFish Extreme (abbreviated SFX, marketed as Nitro), announced on September 18, 2008, which compiles JavaScript into native machine code, eliminating the need for a bytecode interpreter and thus speeding up JavaScript execution.