I am learning React Native but do not understand some syntax used in <https://facebook.github.io/react-native/docs/listview.html#content>

```
 _pressData: ({}: {[key: number]: boolean}),

_genRows: function(pressData: {[key: number]: boolean}): Array<string> {

```

I tried to check if it is new ES6 feature but no luck. I would appreciate if someone can give me a reference to that syntax or explain it here. 

Thank you.