Add `PromiseResult` type to standard lib

[ethanresnick]

Search Terms

promise result type

Suggestion

Typescript already ships with a ReturnType type and a Parameters type, so why not ship with a PromiseResult type:

type PromiseResult<T> = T extends Promise<infer U> ? U : never;

Use Cases/Examples

type Output = PromiseResult<ReturnType<typeof computeSomething>>;

async function computeSomething(input: any) {}
function computeFromResult(input: Output) {}

Checklist

My suggestion meets these guidelines:

• This wouldn't be a breaking change in existing TypeScript / JavaScript code
• This wouldn't change the runtime behavior of existing JavaScript code
• This could be implemented without emitting different JS based on the types of the expressions
• This isn't a runtime feature (e.g. new expression-level syntax)

[DanielRosenwasser]

See #17077, and discussion at

• Design Meeting Notes, 8/4/2017 #17621
• Design Meeting Notes, 8/25/2017 #18155
• Design Meeting Notes, 10/13/2017 #19169
• Design Meeting Notes, 2/9/2018 #21838

Search also for awaited, await, promised, unpromise, and unpromisify on the issue tracker.

[DanielRosenwasser]

Just to clarify, for Promises to work correctly we need a more general awaited type which unwraps PromiseLikes recursively. If we had that, it would subsume this PromiseResult type.

[ajafff]

Just to clarify, for Promises to work correctly we need a more general awaited type which unwraps PromiseLikes recursively.

Sounds like recursive conditional types to me... but that's already discussed somewhere else

[deser]

Any progress here? :)

[westonkjones]

Interested in this as well. The AWS SDK uses this as the return of their promises
https://docs.aws.amazon.com/AWSJavaScriptSDK/latest/AWS/SecretsManager.html#getSecretValue-property

(method) Request<SecretsManager.GetSecretValueResponse, AWSError>.promise(): Promise<PromiseResult<AWS.SecretsManager.GetSecretValueResponse, AWS.AWSError>>

Returns a 'thenable' promise.

[fregante]

Coming in #35998, hopefully