AssemblyScript [1] is an example of a subset of JavaScript being compiled to fast WebAssembly. But it is very static code. If you want to run dynamic JavaScript fast, you need the more sophisticated strategies used by modern JavaScript engines.
[1] https://www.assemblyscript.org/
You mean the client? That'd be an intermediate step to metal. If you want speed, the fewer intermediate steps the better.
Otherwise I don't understand the question.
Why would they?
AssemblyScript [1] is an example of a subset of JavaScript being compiled to fast WebAssembly. But it is very static code. If you want to run dynamic JavaScript fast, you need the more sophisticated strategies used by modern JavaScript engines.
[1] https://www.assemblyscript.org/