satisfies with functions

[kaaax0815]

Suggestion

🔍 Search Terms

satisifes, function

✅ Viability 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. library functionality, non-ECMAScript syntax with JavaScript output, new syntax sugar for JS, etc.)
• This feature would agree with the rest of TypeScript's Design Goals.

⭐ Suggestion

satisifes keyword works with functions

📃 Motivating Example

type AllowedTypes = string | number | boolean | symbol | object | undefined | null;
type LoaderType = () => AllowedTypes;
type LoaderReturn = ReturnType<LoaderType>;

function loader1(): AllowedTypes {
  return 'anything';
}

const value1 = loader1();
//     ^? value1: AllowedTypes 

function loader2() {
  return 'anything';
} satisfies LoaderType

const value2 = loader2();
//     ^? value2: string 

function loader3() {
  loader1()
} satisfies LoaderType
// ^? error: () => void does not satisfy () => AllowedTypes 

💻 Use Cases

In Libaries like Remix you have a loader which has to return something other than void. But there is no good method to check for that
This would check that you return anything, but the frontend is still able to infer what types are returned

[MartinJohns]

Sounds like a duplicate of #52902.

[kaaax0815]

yes this only extends it with arrow functions and the already existing satisfies keyword

[IllusionMH]

Looks more like duplicate of #51556