π Search Terms
return type promise union
π Version & Regression Information
This is the behavior in every version I tried, and I reviewed the FAQ for entries about Promise
β― Playground Link
https://www.typescriptlang.org/play/?#code/C4TwDgpgBAyg9gWwgOTgE2gXigZ2AJwEsA7AcygB8oAFfRQnCAHjyLID4BYAKB4EMcIYgGMoAMwCuI4ITjEo+CMQz4AwgAtCAGzQAKAJQAuGnQQNm8JKgzsoAbx5QnCiMAn55teowB0inHBaAG4QugBEwBB4Yfo8AL48PJLSsvKKyhD4AIJgYLrCmjrpxpYo6BD69oncCbzcAPT1sOpwEjpQoJAFEMIA1lAARhLAUJl0+DiGPI1OACrg0ADkXmaMLAQkpOyLUAxQxHAjAjiEpMR8A1rQwHAdC1CLrJuLProATADMACwArLHc6RUOTygMyGm0en0-xmzlhAD0APw8IA
π» Code
type SomeNode = string | Promise<string>
async function renderChild(): Promise<SomeNode> {
return Promise.resolve("test")
}
function renderApp(children: SomeNode) {
}
// Should typecheck but errors:
// Type 'Promise<string>' is not assignable to type 'string'.(2345)
renderApp(renderChild())
// ^?
π Actual behavior
Argument of type 'Promise<SomeNode>' is not assignable to parameter of type 'SomeNode'.
Type 'Promise<SomeNode>' is not assignable to type 'Promise<string>'.
Type 'SomeNode' is not assignable to type 'string'.
Type 'Promise<string>' is not assignable to type 'string'.(2345)
input.tsx(13, 11): Did you forget to use 'await'?
π Expected behavior
No typechecking error
Additional information about the issue
Nodes in React are typed as type Node = AwaitedNodes | Promise<AwaitedNodes>. However, the return type of async Components is Promise<Node>. But TypeScript will not consider this a Component because Promise<Node> is not assignable to Node even though it should be since at runtime Promise<Promise<T>> will never be observable and always collapse to Promise<T>.
Users can either fix this by using an unwieldy Promise<Awaited<ReactNode>>.
We can also fix this at the type level by allowing Node | Promise<Node> as the return type. Though I suspect this just pushes the issue one Promise-wrapping level away when TypeScript could collapse wrapped Promises automatically.
Original issue: vercel/next.js#67365
π Search Terms
return type promise union
π Version & Regression Information
This is the behavior in every version I tried, and I reviewed the FAQ for entries about Promise
β― Playground Link
https://www.typescriptlang.org/play/?#code/C4TwDgpgBAyg9gWwgOTgE2gXigZ2AJwEsA7AcygB8oAFfRQnCAHjyLID4BYAKB4EMcIYgGMoAMwCuI4ITjEo+CMQz4AwgAtCAGzQAKAJQAuGnQQNm8JKgzsoAbx5QnCiMAn55teowB0inHBaAG4QugBEwBB4Yfo8AL48PJLSsvKKyhD4AIJgYLrCmjrpxpYo6BD69oncCbzcAPT1sOpwEjpQoJAFEMIA1lAARhLAUJl0+DiGPI1OACrg0ADkXmaMLAQkpOyLUAxQxHAjAjiEpMR8A1rQwHAdC1CLrJuLProATADMACwArLHc6RUOTygMyGm0en0-xmzlhAD0APw8IA
π» Code
π Actual behavior
π Expected behavior
No typechecking error
Additional information about the issue
Nodes in React are typed as
type Node = AwaitedNodes | Promise<AwaitedNodes>. However, the return type of async Components isPromise<Node>. But TypeScript will not consider this a Component becausePromise<Node>is not assignable toNodeeven though it should be since at runtimePromise<Promise<T>>will never be observable and always collapse toPromise<T>.Users can either fix this by using an unwieldy
Promise<Awaited<ReactNode>>.We can also fix this at the type level by allowing
Node | Promise<Node>as the return type. Though I suspect this just pushes the issue one Promise-wrapping level away when TypeScript could collapse wrapped Promises automatically.Original issue: vercel/next.js#67365