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?
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