I need to create an offline application with HTML and JS, that can edit and write files (local application folder, but NOT sandboxed browser local storage).
Is there any way to do this, in a reasonably lightweight and straightforward way? Researching this question gave me the following overview of available options - Electron is gigantic, Chrome Apps are a discontinued project, W3C FileSystem API was abandoned, W3C File API is very restrictive.
There have been multiple attempts to create an SDK on top of Node.js for building desktop applications powered by modern HTML5 / CSS / JS frontends. AppJS and its offshoot Deskshell come to mind. And then there's Node-Qt, which is built on top of the Qt platform.
Unfortunately, none of these projects reached sufficient levels of maturity for me to recommend them, and they all seem to have been abandoned years ago. NW.js is the only alternative to Electron that I can think of that, at the moment, reached a decent level of maturity and is still being maintained.
That is, at least, if your focus is a desktop computer. For applications intended to be run on mobile devices, you might want to take a look at NativeScript, React Native or Tabris.