JSC.js is a JavaScript engine which can run on top of you browser's JavaScript engine. It's based on the JavaScript engine (JavaScriptCore) of WebKit and compiled to wasm with emscripten. Therefore, if you're using Safari, you are literally running its JavaScript engine on top of itself.
The size of JSC.wasm is around 4MB (compressed js and mem file).
Demo: Link
ScreenShot

Build
Preparation
- install emscripten
 - install python, ruby, ninja, etc.
 - run 
build/gn/download.batto download latestgn.exe. - run 
Source/JavaScriptCore/DerivedSources/gen.batto generate derived sources. 
Build with gn
> gn gen out --args="target_os=\"wasm\""
> ninja -C out
Build test shell on Windows
> gn gen out --args="target_os=\"win\""
> ninja -C out