Type for function with specified parameters but unspecified return type

[subtle bramble]

Is it possible to use type to specify a type of function where the parameters are of defined types but we do not specify the return type. Yet typescript can still infer the return type as it would with an untyped function?

https://www.typescriptlang.org/play?#code/C4TwDgpgBAcgrgWwGJwHYGMoF4oAoAeAXFKogEYQBOAlNgHxQCGqIAUK+gPaoDOwTheMjSYcBWlgb4oDAKwdufKGUGIUGbHnwSGAbQB0hgIKVKjEOP0BrCCB65qAXX0ArTgEtUuAOQAab9QKvPzoqsIaYtr0UNIAVFAATOxcwVBGmoy4AMy0APS5oJA86JTuYPyeAGZUPFDAABaM-OnutQBEzCBt+lAAEpwA7nWcUAiMNlDu-GgAJjXAzDOTFe1knJwANhDMbQD8QUoAQppkuAAseQXgEMWl5ZOo1ZS1DU1Qx61QHSzdfYPDo3G0CmUFm80Wy0m7T4pVQAHM9gd+ABhTToXCyS6FG4lMoVR41OqNFFQr6dX79IbAEZjCYgsHPBaoJYgz5tUgICiURFAA

[fluid coral]

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

zaceno#7945

Playground Link

Preview:```ts
type NumFunc = (x: number) => any

const a:NumFunc = (x) => x > 5
const b:NumFunc = (x) => [...Array(x).keys()].join(',')
const c:NumFunc = (x) => x * 2

const A = a(3) //typescript infers that A is "any". How to make it understand it is "boolean"?
const B = b(4) //typescript infers that B is "any". How to make it understand it is "string"?
...```

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[vernal spade]

just don't write a manual type for it

[fluid coral]

That_Guy977#5882

Playground Link

Preview:```ts
type NumFunc = (x: number) => unknown

const a = ((x) => x > 5) satisfies NumFunc
// ^?
const b = ((x) => [...Array(x).keys()].join(',')) satisfies NumFunc
// ^?
const c = ((x) => x * 2) satisfies NumFunc
// ^?

const A = a(3) //typescript infers that A is "any". How to make it understand it is "boolean"?
...```

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

[vapid spire]

also works with any, but don't use any if avoidable

[vernal spade]

yeah that's a better way

[subtle bramble]

satisfies huh. Haven't encountered that keyword before. Is that new? Gonna go read up

[vapid spire]

it's pretty new yeah

[subtle bramble]

Cool cool cool. Thanks!

First paragraph of the docs for satisfies:

TypeScript developers are often faced with a dilemma: we want to ensure that some expression matches some type, but also want to keep the most specific type of that expression for inference purposes

That is a better, more general wording of precisely the issue I am having.

So new, and it already works in JSDoc as well. Brilliant

[subtle bramble]

forgot: !resolved