How to typecast in typescript-CodePudding

I have the following loadMore function that returns Promise<Project[] | Folder[] | undefined>.

const items = await loadMore();

I want to cast the type of items to Folder[]. How do I do that using typescript?

CodePudding user response：

This is another way of doing it :

const items: Folder[] = loadMore()

CodePudding user response：

Using a type assertion:

const items = await loadMore() as Folder[];

However, it's safer to discriminate the union.

CodePudding user response：

I would offer that it's better to discriminate the items of the array with the following inline type-discriminator used as the predicate in Array.filter

const folderItems = items?.filter(
    (v: Folder | Project): v is Folder => /* is v really a Folder? true/false */
);

to give you either an array that you can be sure contains only Folder items, or undefined if items is undefined.