What means ":?" average value in javascript source code

Question

What does something like that mean?

_scrollViewRef: (null: ?ScrollView)

https://github.com/facebook/react-native/blob/master/Libraries/Components/ScrollView/ScrollView.js#L439

or that:

watchID = (null: ?number);

https://www.tutorialspoint.com/react_native/react_native_geolocation.htm

my confusion in this part is " :? "
how do these codes work? what does it mean?