I would suggest you to learn JavaScript because it is truly cross platform language. You can build web, mobile and desktop applications.
You can build desktop application with ElectronJS. Interesting thing is you can even use almost all JavaScript libraries and frameworks with electron. Currently, I am developing a Desktop app with Electron and using VueJS for single application functionality and state management.
Ionic and React Native:
You can use either Ionic or React Native to develop mobile applications for both iOS and Android. What else you ask for? JavaScript is everywhere. As of I know, there are many companies use React for web and React Native for mobile and share 90% of same code base.
Web:
Web is overflowing with front-end JavaScript libraries and frameworks. For backend, Node.js The popular frondend libraries and frameworks are:
There is no better time than now to learn JavaScript.