Google has released a JS unit testing framework based on V8.
Some of its features include:
Extremely fast startup and execution time, without needing to run a browser.
Clean, readable output in the case of both passing and failing tests.
An optional browser-based test runner that can simply be refreshed whenever JS is changed.
Style and semantics that resemble Google Test for C++.
A built-in mocking framework that requires minimal boilerplate code (e.g. no $tearDown or $verifyAll calls), with style and semantics based on the Google C++ Mocking Framework.
A system of matchers allowing for expressive tests and easy to read failure output, with many built-in matchers and the ability for the user to add their own.