Inferring function parameters from the return type

🔍 Search Terms

"inferring parameters types", "infer input from output types"

✅ Viability Checklist

• 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 isn't a request to add a new utility type: https://github.com/microsoft/TypeScript/wiki/No-New-Utility-Types
• This feature would agree with the rest of our Design Goals: https://github.com/Microsoft/TypeScript/wiki/TypeScript-Design-Goals

⭐ Suggestion

I would like to infer the types of the parameters of a function based on the return type. I have tried a few patterns and I don't think this is currently possible.

📃 Motivating Example

const inferReturnValuesAsKeys = <
  T extends (arg: keyof ReturnType<T> & string) => Record<string, number>,
>(
  t: T,
) => t;

/**
 * @todo arg should be typed as 'a' instead of 'string'
 */
inferReturnValuesAsKeys((arg) => ({ a: 5 }));
// ^?

💻 Use Cases

1. What do you want to use this for?

Helpful for strict typings and intellisense.

3. What shortcomings exist with current approaches?

None give a strict enough type.

5. What workarounds are you using in the meantime?

I don't have the type safety that I want.

Start with the motivating TypeScript example and review the linked Design Goals and existing inference behavior. Determine whether the compiler can infer the callback parameter as the return object's key while preserving JavaScript output and existing type-checking behavior; done means the example has the requested type and suitable tests cover it.