In React Native Example Code, you'll find at some files the type statement, which encapsulates 4 properties (I'd like to guess), where the last two ones are suffixed with question marks.
type MapRegion = {
latitude: number,
longitude: number,
latitudeDelta?: number,
^============ What are these...
longitudeDelta?: number,
}; ^===========...question marks for?
That is flow, a transpiler language that adds static typing to JavaScript.
type MapRegion = {
latitude: number,
longitude: number,
// This property is nullable
latitudeDelta?: number,
// This property is nullable
longitudeDelta?: number,
};
// The following does not cause a compilation error
/* @flow */
var a:MapRegion = {
latitude: 1,
longitude: 3
};
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