Promise#finally's callback doesn't accept Promise<void> callback, unlike Promise#then #44980
Description
lib Update Request
Configuration Check
My compilation target is ES2018 and my lib is the default.
Missing / Incorrect Definition
Promise#finally
Sample Code
export async function closeDbConnection(): Promise<void> {
return Promise.resolve();
}
Promise
.resolve()
.finally(closeDbConnection);
The suggested change is changing lib/lib.es2018.promise.d.ts from
finally(onfinally?: (() => void) | undefined | null): Promise<T>
to
finally(onfinally?: (() => void | PromiseLike<void>) | undefined | null): Promise<T>
The current typing, without the PromiseLike, suggests that the runtime will not wait for the onfinally callback to resolve if it's a Promise, however, it does. This can be seen in the console when test changes from pending to 123.
> test = Promise.resolve(123).finally(() => new Promise(resolve => setTimeout(resolve, 5000)))
Promise { <pending> }
> test
Promise { <pending> }
> test
Promise { <pending> }
> test
Promise { <pending> }
> test
Promise { <pending> }
> test
Promise { <pending> }
> test
Promise { <pending> }
> test
Promise { 123 }
Additionally, for consistency between #then and #finally, the current onfinally definition doesn't match Promise#then in lib/lib.es5.d.ts where onfullfilled returns TResult1 | PromiseLike<TResult1>.
interface Promise<T> {
/**
* Attaches callbacks for the resolution and/or rejection of the Promise.
* @param onfulfilled The callback to execute when the Promise is resolved.
* @param onrejected The callback to execute when the Promise is rejected.
* @returns A Promise for the completion of which ever callback is executed.
*/
then<TResult1 = T, TResult2 = never>(onfulfilled?: ((value: T) => TResult1 | PromiseLike<TResult1>) | undefined | null, onrejected?: ((reason: any) => TResult2 | PromiseLike<TResult2>) | undefined | null): Promise<TResult1 | TResult2>;
Documentation Link
Suggests that .finally() is implemented similar to .then(result => Promise.resolve(onFinally()).then(result)).