In D15056#416323, @angelika wrote:

$ArrayType is an undocumented built-in type from flow: https://github.com/facebook/flow/blob/34098d9ab16e9470c597e3ed4d40bbc7dd1da8d7/prelude/prelude.js#L34C7-L34C16
but react-native uses anyway it to type `getItemLayout: https://github.com/facebook/react-native/blame/504cf3e9330285ba8995dd938756d2ea13ffa28d/packages/react-native/Libraries/Lists/FlatList.js#L96

That makes sense! So it seems like $ArrayLike is a supertype of any array type, and we shouldn't have any expectations about a concrete type in our code.