V8 is Google's open source Javascr ipt engine. V8 is written in C++ and is used in Google Chrome, the open source browser from Google. V8 implements ECMAscr ipt as specified in ECMA-262, 3rd edition, and runs on Windows XP and Vista, Mac OS X 10.5 (
SpiderMonkey is Mozilla's Javascr ipt engine written in C/C++. It is used in various Mozilla products, including Firefox, and is available under the MPL2.