You can use anything you can use from Javascript. E.g., https://www.npmjs.com/package/memfs can be used with https://www.npmjs.com/package/@wapython/wasi to provide a ramdisk, and similarly you can make it persist to indexdb or localStorage. This sort of thing is also built into emscripten. You can see this in action at https://jupyter.org/try-jupyter/lab/

It's also possible (on Google Chrome) to use browser API's to directly access your host computer's filesystem (see https://developer.mozilla.org/en-US/docs/Web/API/File_System...).