I don't understand why the downvotes , JS is one of the most target languages. Elm,GHJS(Haskell),Purescript,Clojurescript and supersets like Typescript which come with their own toolchains.
Also Javascript itself is frequently compiled down to an older version of Javascript.
Asm.js was exactly what your talking about.
You can think of webassembly more like a common standard akin to the JVM specification. Anyone can implement the spec.
It's a formal definition of what people are doing with JS.