Asyncify lost the general return type

[linonetwo]

I have a 'get' method like this:

And I Asyncify it through this:

/**
 * To call services that is located in main process, from the renderer process, we use IPC.invoke, so all method should now promisify
 * Note this type only promisify methods that return things, not methods that returns observable.
 */
type AsyncifyProxy<OriginalProxy, K extends ConditionalKeys<OriginalProxy, Function> = ConditionalKeys<OriginalProxy, Function>> = {
  [P in K]: Asyncify<OriginalProxy[P]>;
};

Then the result lost the type:

I can see it don't allow the general type now:

it used to know the return type based on the input...

Is it possible to solve this?

https://github.com/sindresorhus/type-fest/blob/main/source/asyncify.d.ts

[sindresorhus]

// @papb

[linonetwo]

Playground here:

Playground Link

[papb]

Oops, sorry for the delay, I missed this notification.

Hmm... This is a TypeScript limitation, unfortunately. The simple act of writing (...args: Parameters<F>): ReturnType<F> already "settles" all generics into their widest type (usually unknown).

I tried for some time to find another workaround to keep at least the most common generic usages but I couldn't.

Here are some related TS issues: microsoft/TypeScript#39594, microsoft/TypeScript#10247, microsoft/TypeScript#37835.

This problem also happens with SetReturnType.

Should we add a note to the docs that it does not work with functions with generic types?

[linonetwo]

Thank you, I also confirmed today from TS member, this is a design limintation microsoft/TypeScript#42706

Seems there is no official workaround currently.