[webkit-dev] question about javascript bindings

I'm a beginner to WebKit domain(linux qt platform). I'm try to understand how to port another JavaScript Engine to webKit as chrome did. Now I'm confused by the code of *webcore/binding/js*, I found the function defined in webcore/bindings/js/*Custom*.cpp never be used when debugging the webkit using a html page with javascript code, So it's hard to me to understand this part. Anyone can give me some helps? Any documents or background knowledge are a precondition to learn this part. Thanks malin