The best (and well, simplest way) to explain what nw.js is...it's kind of a shell/app runtime based on Chromium and node.js, so assuming that and the very likely-correct premise that it's just Chromium and node.js running in a shell so it likely stores 'savedata' (whether say said 'savedata' is methoded through the system save/load or via localstorage and dictionaries/arrays) in whatever is equivalent to a browser's cache, but a clear note here that the 'cache' for games with Construct 2 is seperate from a browser's cache but is the closest parallel, in terms of describing it.
Given that, the savedata is likely stored internally in nwjs' exe.