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Extract function parameters or change function return type in mapped types

See original GitHub issue

Search Terms

extract function parameters change function return type


Allow mapped types to change the return type of a function

Use Cases

I am writing a mocking framework. My system under test might be like this:

    myClassFunction(paramOne: string, paramTwo: number): string{}

but in my mocking framework I want to do this:

    .withParameters("paramOneValue", 123)

to do the withParameters I need to type a function with the same parameters as defined on my class but with a different return type.


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. new expression-level syntax)

Issue Analytics

  • State:open
  • Created 5 years ago
  • Reactions:8
  • Comments:5 (1 by maintainers)

github_iconTop GitHub Comments

Keyvescommented, Oct 26, 2018

like this?

type MapResultToPromise<T> = T extends (...args: infer U) => infer R ? (...args: U) => Promise<R> : T;
Cerberuscommented, Feb 15, 2019
type ArgumentTypes<T> = T extends (... args: infer U ) => infer R ? U: never;
type ReplaceReturnType<T, TNewReturn> = (...a: ArgumentTypes<T>) => TNewReturn;

I found this on stackoverflow. It’s very useful. ref:

And I’ve tried to apply Proxy to object.

type Proxify<T> = { [P in keyof T]: ReplaceReturnType<T[P], Proxify<T>> }

function proxify<T>(o: T): Proxify<T> {
	return new Proxy(o, handler)

So then we get a chain-proxy object as you want

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