A tool called Emscripten provides a runtime environment. You can include things from that, like a virtual filesystem. That said, a lot of WebAssembly code doesn't have a dependency on file APIs. For example, you can check out the web-dsp project: https://github.com/shamadee/web-dsp

If you're looking for an intro, I wrote a series of posts on WebAssembly (there are 5 links at the bottom of this intro): https://hacks.mozilla.org/2017/02/a-cartoon-intro-to-webasse...