I don’t know if it needs to be though I suspect it does (I went by other plugins that work)
i.e. register_asset “auto-magically” looks for an “assets” sub-folder, so it exists in the folder structure but you don’t need to (shouldn’t) have that in the register_asset path
Also, I guess you could name the folder “javascript” but every example I’ve seen uses the plural “javascripts”
I imagine it is only because the folder often contains more, but it might also have something to do with the “auto-magically” loading of es6 files.
Finally, I named it “progress” but you should probably use a name more meaningful to what it’s doing. (though AFAIK any name should work)