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improv: update types #9

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improv: update types #9

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improv: update types 
Use infer to retrieve function return type
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@Eywek
Eywek authored Dec 30, 2019
commit fb9b433c03009a66ce1c5f9a050ff570fa15a0ae
11 changes: 6 additions & 5 deletions src/index.ts
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Original file line number Diff line number Diff line change
@@ -1,12 +1,13 @@
export type AWAIT_OF_RETURN_TYPE = [any?, Error?];
type Unpromisify<T> = T extends Promise<infer R> ? R : T
type ReturnType<T> = T extends (...args: any[]) => infer T ? Unpromisify<T> : any

export function of(promise: Promise<any>): Promise<AWAIT_OF_RETURN_TYPE> {
export function of<T> (promise: Promise<ReturnType<T>>): Promise<[undefined, Error] | [ReturnType<T>]> {
return Promise.resolve(promise)
.then((r: any): AWAIT_OF_RETURN_TYPE => [r])
.then((r: ReturnType<T>): [ReturnType<T>] => [r])
.catch(
(e: Error | undefined | null): AWAIT_OF_RETURN_TYPE => {
(e: Error | undefined | null): [undefined, Error] => {
if (e === undefined || e === null) {
return [e, new Error("Rejection with empty value")];
return [undefined, new Error("Rejection with empty value")];
}

return [undefined, e];
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