How to get the inferred return type when using a return signature?

[pure tree]

It's all in the title

Basically, I want to access the inferred return type from a function, while also using a type. Here's a minimal example:

[cosmic wyvern]

oodavid#0

Playground Link

Preview:```ts
/** The Goal
*

• 1. To use a type for the function
• 2. To access the inferred promised return type for the function
     */
     interface CallbackParams {
     prop: string;
     }
     interface CallbackResponse {
     [key: string]: string;
     }
     type CallbackSignature = (params: CallbackParams) => Promise<CallbackResponse>;
     ...```

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[craggy kraken]

you can change the declaration of the function so that it is not blocked by the generic but still meets the requirements

const myCallback = async ({ prop }:CallbackParams) => {
   return {
       hello: 'world',
       myProp: prop,
   };
}

[cosmic wyvern]

m.guiu#0

Playground Link

Preview:ts ... const myCallback = async ({prop}: CallbackParams) => { return { hello: "world", myProp: prop, } } ...

[craggy kraken]

In this way you can obtain the type

[pure tree]

I considered that, but I want the return type to match CallbackResponse interface.

ie: this would break my code:

const myCallback = async ({ prop }:CallbackParams) => {
    return {
        hello: 'world',
        myProp: prop,
        today: new Date(), // strings only please!
        good: true, // strings only please!
    };
}

[barren yarrow]

you could use satisfies

this sounds like a specialized case of "check against constraint but infer more specifically"

const fn = () => ["a"] satisfies () => unknown[]

!ts fn

[cosmic wyvern]

const fn: () => string[] /* 1:7 */```

[pure tree]

Thanks, @barren yarrow I'll see if I can make that work and update this thread 🙂

[pure tree]

Thanks again, all sorted.

I had to wrap the function declaration in (parenthesis) in order to add the satisfies statement.

Here's my final code:

https://www.typescriptlang.org/play/?#code/PQKhAIBUAsFNwOIHsCGAbAsAKHBbvwBGAOiiXAFcBneFcAFwE8AHeAMyQCcG5w2KAdgGN6ASyQDw+CACZSkciiFDYVKj3iiBbWJ06wAJuGackAW1E0j++hU6SmrPlw19BI8QOnBsW+rrYleABhdDQAIyUAawAFFE4UM3UAb3xwdJMkZgAucCp6Ti0AcwBubABfbF8Bf05AlXBQtAjogCVVZgkacFScdPAAbSjYRlz8woEigF0xguKyrEqsbEcQsMihKIBlUSKBFFt9cABecAAKZnjEqlymls24hKSAShOAPnAY0wsaAB47jZRdpUToCGhvBbYYDAKDQSzgaAodR0ABu6FE1lghwcLFg2CEXXo4DMjAB0RO5yRjGE52SxlMzHA5Vexw+vX64Bsdkk7I5-TgzSQuQA5AB3LhoAzCgA0aT5xMYXyyuUyzFlfXl0PA4SQSDQuUCaBo6o5S2ZeQOljYolUjXW0R2ewOdlgkKwWpg8IJelgIjQjARSPASDs4C0Oh9mOxDFxK1x4AAsqT7Zt2tjIPHTgBBUUoUT+Ay-NPcjOsX6rJBsBVkzZvCFAA

[cosmic wyvern]

@pure tree Here's a shortened URL of your playground link! You can remove the full link from your message.

oodavid#0

Playground Link

Preview:```ts
/** The Goal
*

• 1. To use a type for the function
• 2. To access the inferred promised return type for the function
     */
     interface CallbackParams {
     prop: string;
     }

interface CallbackResponse {
[key: string]: string;
}

type CallbackSignature = (params: CallbackParams) => Promise<CallbackResponse>;
...```

You can choose specific lines to embed by selecting them before copying the link.