What is the method for referencing a subtype within an established type?

When working with React-native, I came across a component called FlatList which includes a property known as ListHeaderComponent. My question is how to specify the type of this property without having to manually copy and paste the original type.

Currently, my code looks like this:

export function AccountMediaGalleryComponent({
  style,
  ListHeaderComponent,
  ListHeaderComponentStyle,
}: {
  style: StyleProp<ViewStyle>;
  ListHeaderComponent?: React.ComponentType<any> | React.ReactElement | null;
  ListHeaderComponentStyle?: StyleProp<ViewStyle>;
}): JSX.Element {

I had hoped to do something like:

ListHeaderComponent?: typeof FlatList.ListHeaderComponent

However, this did not compile successfully.

Answer №1

While I may not be an expert in react-native, my understanding is that if you have a value of type ListComponent, it likely contains a property named props which in turn includes a property named ListHeaderComponent, the type you are seeking.

To access this type, you can utilize indexed access types. Think of it as extending bracket notation for accessing properties to the type level; if a value t has type T, and another value k has type K, then t[k] will have type T[K]. (Note that dot notation cannot be used for this purpose to avoid ambiguity in the type system; refer to microsoft/TypeScript#30815 for more details).

In essence, by indexing into a value of type FlatList with a literal type "props", and further indexing into that with another literal type "ListHeaderComponent", you arrive at the desired type:

type ListHeaderComponent = FlatList["props"]["ListHeaderComponent"];

resulting in the type you are looking for:

/* type ListHeaderComponent = 
     React.ComponentType<any> | 
     React.ReactElement<any, string | React.JSXElementConstructor<any>> |
     null | undefined */

For a hands-on demonstration with code play around, you can follow this Playground link.

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