Read JavaScript native code

The reason Chrome or Firefox says that the code is native is that it really is native - the WebKit and Firefox developers have coded up that functionality in C or C++, not JavaScript. However, if you want to see the actual code, you can look at the source repositories for Chromium and Firefox.

Not within the JavaScript environment, but you can view the source for the open-source implementations.

Google V8: http://code.google.com/p/v8/source/browse