No, a different subset. JavaScript is unsuited to WebAssembly because of its dynamic nature; this project is taking the type system of TypeScript and using it to allow compilation to WebAssembly, but the dynamic parts of TypeScript/JavaScript are disallowed.

Just as asm.js is a subset of JavaScript that can be optimised more successfully, AssemblyScript is a subset of TypeScript that can be optimised more successfully.

You mean other than C/C++ where you cab take advantage of 30 years of libraries ?

http://kripken.github.io/emscripten-site/

Yes, other than C++.

Rust has a WASM target as well as ASM.JS.

Although, it seems as though it compiles to ASM.JS and then compiles that ASM into WASM, so likely anything that compiles to ASM.JS can also then be further compiled into WASM.