JsFs is a pure JavaScript implementation of an Emscripten-compatible filesystem. It is based heavily off of Emscripten's MEMFS.
JsFs is licensed under the MIT license. It incorporates code from Emscripten, which is also licensed under the MIT license.
Emscripten ships MEMFS as code inside Emscripten modules. Using MEMFS without first instantiating a module isn't really possible. JsFs permits creating file systems and populating them with files without this dependency. JsFs file systems can be mounted inside Emscripten when convenient.