Inferring function parameters from the return type

[kyscott18]

🔍 Search Terms

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

✅ Viability Checklist

• [X] This wouldn't be a breaking change in existing TypeScript/JavaScript code
• [X] This wouldn't change the runtime behavior of existing JavaScript code
• [X] This could be implemented without emitting different JS based on the types of the expressions
• [X] This isn't a runtime feature (e.g. library functionality, non-ECMAScript syntax with JavaScript output, new syntax sugar for JS, etc.)
• [X] This isn't a request to add a new utility type: https://github.com/microsoft/TypeScript/wiki/No-New-Utility-Types
• [X] 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.

[jcalz]

essentially a duplicate of #49618

[3rd]

Spent a few hours trying to work around this without success, adding my use case:

// would love for this to work: function create<T>(fn: (root: T) => T): T {}

const example = create((root) => ({
  static: 10,
  computed: () => root.static + 5,
}));

example.computed() // 15