React Native [0]. Your view-specific code still has to be done per-platform, but your core logic can be shared, and all of it is written with Javascript using React as the framework. The JS creates and controls native elements, with the native parts mostly abstracted away.
I'm referring to native apps on iOS and Android as the code commonality is huge, with just a small number of platform specific components. The biggest surprise though was how much I could port out for a web version (and Electron based Windows and Mac applications)
[0] https://facebook.github.io/react-native/