Just getting started on Electron myself. Used by Atom, Slack, Visual Studio... It's worth a look ;) Electron - Build Cross-Platform Desktop Apps With HTML, JS, CSS http://electron.atom.io/
Electron only works for desktop applications and probably needs a js-framework to work good. I think that React + Electron looks like a nice combo and then you can use React Native to create Android and iPhone apps also, probably sharing a lot of code between all versions.
To be fair "just" might be over simplifying things a bit. I've been doing this for an application and thinking that it actually would be a benefit to have a different code bases for the desktop and mobile editions. It can be a bit challenging to design for both targets at once.