The FileSystemObserver interface notifies websites of changes to the file system. Sites observe changes to files and directories, to which the user has previously granted permission, in the user's local device (as specified in WICG/file-system-access) or in the Bucket File System (as specified in whatwg/fs), and are notified of basic change info, such as the change type.
Currently, for a given agent to know about modifications to the file system - made either by itself or from some external context - it must poll the file system to detect changes. This is inefficient and does not scale well. The FileSystemObserver interface makes it easier for a website to be notified of changes to the file system.
Docs: https://docs.google.com/document/d/1d6YoPvk0rdNBQaZcoFP-V5u4CBjgahvmnVsZpbn0Gmo/edit?usp=sharing
Samples: https://developer.chrome.com/blog/file-system-observer#demo
Explainers: https://github.com/whatwg/fs/blob/main/proposals/FileSystemObserver.md