Which programming language or framework is best for building a cross-platform app with a single codebase?

Don't forget about Nx monorepo! It’s a great way to manage projects across React, React Native, and Node. If you’re proficient with JavaScript or TypeScript, this could streamline your development process.

If you want a solid overlap between platforms while still using JavaScript, try combining Electron with React Native. You won't have a complete single codebase, but you can share a lot of your code, probably around 80%. Plus, there are more resources available for JavaScript.

JavaScript is probably your best bet. You can create a web version, use React Native for mobile apps, and even Electron for desktop applications. It's all built on one language, so you'll be able to manage everything from a single codebase.

I suggest looking into Flutter. It lets you build for multiple platforms from one codebase, but you'll need to get familiar with Dart and the Flutter framework. It might feel like a bigger commitment compared to just using JavaScript, but the results can be rewarding.