WASM and JS are both JIT compiled in all major browsers which means that they compile to the same kind of native code that C and C++ do, they just do so as the program is running rather than in advance.
What's even better about WASM than JS in this case is that it can be compiled as it is being loaded. With JS, the entire file needs to be downloaded before being executed, but that's not the case for WASM, resulting in even more performance improvements.
