Infer generic type from ReturnType

[rdeneau]

I thought I could answer this stackoverflow question with a generic factory function but it fails:

function createItem<T>(value: T) {
    return { value };
}

type ItemV1a = ReturnType<typeof createItem>;  // KO: { value: {}; }
type ItemV1b<T> = ReturnType<typeof createItem>;  // KO: <T> ignored. Still { value: {}; }. Expected: { value: T; }
type ItemV1c<T> = ReturnType<typeof createItem<T>>;  // Compile error

const createItemLamdba = <T>(value: T) => ({ value });

type ItemV2a = ReturnType<typeof createItemLamdba>;  // KO: { value: {}; }
type ItemV2b<T> = ReturnType<typeof createItemLamdba>;  // KO: <T> ignored. Still { value: {}; }
type ItemV2c<T> = ReturnType<typeof createItemLamdba<T>>;  // Compile error

Note : I don't see a way to use the infer keyword.

Expected behavior:

Having a generic type returned by ReturnType type, with a special syntax if necessary.

Playground Link:

https://www.typescriptlang.org/play/index.html#src=function%20createItem%3CT%3E(value%3A%20T)%20%7B%0D%0A%20%20%20%20return%20%7B%20value%20%7D%3B%0D%0A%7D%0D%0A%0D%0Atype%20ItemV1a%20%3D%20ReturnType%3Ctypeof%20createItem%3E%3B%20%20%2F%2F%20KO%3A%20%7B%20value%3A%20%7B%7D%3B%20%7D%0D%0Atype%20ItemV1b%3CT%3E%20%3D%20ReturnType%3Ctypeof%20createItem%3E%3B%20%20%2F%2F%20KO%3A%20%3CT%3E%20ignored.%20Still%20%7B%20value%3A%20%7B%7D%3B%20%7D%0D%0Atype%20ItemV1c%3CT%3E%20%3D%20ReturnType%3Ctypeof%20createItem%3CT%3E%3E%3B%20%20%2F%2F%20Compile%20error%0D%0A%0D%0Aconst%20createItemLamdba%20%3D%20%3CT%3E(value%3A%20T)%20%3D%3E%20(%7B%20value%20%7D)%3B%0D%0A%0D%0Atype%20ItemV2a%20%3D%20ReturnType%3Ctypeof%20createItemLamdba%3E%3B%20%20%2F%2F%20KO%3A%20%7B%20value%3A%20%7B%7D%3B%20%7D%0D%0Atype%20ItemV2b%3CT%3E%20%3D%20ReturnType%3Ctypeof%20createItemLamdba%3E%3B%20%20%2F%2F%20KO%3A%20%3CT%3E%20ignored.%20Still%20%7B%20value%3A%20%7B%7D%3B%20%7D%0D%0Atype%20ItemV2c%3CT%3E%20%3D%20ReturnType%3Ctypeof%20createItemLamdba%3CT%3E%3E%3B%20%20%2F%2F%20Compile%20error%0D%0A

[jack-williams]

I think this is related to this issue: #22617

In particular I would read this comment by Anders that explains the design limitation.

[AnyhowStep]

My workaround for such cases is, (I'm on my phone so I may make typos)

type CreateValueResult<T> = (
    { value : T }
);
function createValue<T> (value : T) : CreateValueResult<T> {
    return { value };
}

So, when I need to use the return type of the generic function, I just use CreateValueResult<T>

No need to introduce new syntax. No need for a new feature. You do lose the return type inference, though.

---

In my projects, I have many generic functions that compose each other and TS' inference isn't always accurate. So, I have the type declarations to explicitly type them.