On NativeScript Windows (packaged WinUI/MSIX x64 app), the synchronous file APIs in @nativescript/core are non-functional while their async counterparts work:

• File.fromPath(p).readTextSync() returns the string "undefined" rather than file contents (feeding it to JSON.parse throws "undefined" is not valid JSON).
• File.fromPath(p).writeTextSync(text) returns without throwing but does not modify the file. Subsequent calls in the same process also report success without writing — the file keeps its initial contents regardless of write count.

File.readText() / File.writeText() (async) perform real IO on the same paths.

Evidence: a packaged app calling writeTextSync 18 times per boot (instrumented counter, no thrown errors) produced a file containing only the first write's data; readTextSync of that file threw a JSON parse error on "undefined". This caused a preferences-persistence feature to silently lose every write — a real consumer-facing data-loss path for any app relying on the sync APIs (e.g. settings stores, caches).

Environment: Windows 11 x64 (10.0.26200.9711), packaged MSIX, NativeScript Windows runtime current as of Nov 2026.