$ObjMap support

[rattrayalex]

I'm not sure if it's possible (either way might be good to document).

[piotrwitek]

I would gladly add that to the API, although I'm not sure I'll have time to work on this any time soon.

If anyone would want to work on this, PR'a are welcomed.

For any contributor here is a use-case example to clarify what this type should achieve:

type $ObjMap<T extends Record<string, any>,  MapFn extends ((...args: any[]) => any)> = {
    [P in keyof T]: MapFn<T[P]>; // MapFn is executed on each value of an object
}

type MapToReturnType<T> = T extends ((...args: any[]) => any) ? ReturnType<T> : never;

const o = {
  a: () => true,
  b: () => 'foo'
};

function mapValues<T extends {[key: string]: (...args: any[]) => any}>(o: T): $ObjMap<T, MapToReturnType> {
	return Object.values(fn => fn());
}

const result = mapValues(o) // result.a is boolean, result.b is 'foo'

result.a // boolean
result.b // 'foo'

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[zetamorph]

Hi,

I looked around and went through some related TypeScript issues and it seems that this is not possible with TypeScript at the moment.
Something like this microsoft/TypeScript#6606 would have to exist in TypeScript to strictly infer the types of all mapped properties of the object.

It also seems that this change in the TypeScript compiler will not land for a long time, as can be read here: microsoft/TypeScript#26043 (comment)

[stereobooster]

type ObjMap<O extends {}, T> = { [K in keyof O]: T };

works in TS 3.1 (probably in 2.9 too) bcherny/flow-to-typescript#11

Update: this is not the same as original $ObjMap, I thought about different use case

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@malash has submitted a pull request. See it on IssueHunt

[piotrwitek]

I wanted to refer to some proposals posted here that are different from original $ObjMap in Flow.
The primary use case we would like to solve with this type is to iterate over properties of object and execute a function on each value to get the result. Obviously this has to be done on a type-level only.

I know TS doesn't allow this operation right now (please read #18 (comment) above) but we don't want to ship half-working solution as it would be confusing to people try to use it as explained above and flooding us with complains that it doesn't work.

This issue will stay open here waiting until TS allow this kind of feature.

[captain-yossarian]

Hi @piotrwitek ,
Do you mean something like this

type $ObjMap<T extends Record<string, any>> = {
    [P in keyof T]: T[P] extends (...args: any[]) => any ? ReturnType<T[P]> : never;
}

[piotrwitek]

Hey @captain-yossarian, I have added the clarification about this type, please check this comment: #18 (comment)

[captain-yossarian]

@piotrwitek is it possible to run tests on windows? Every time I'm running npm run test:update, I'm receiving:

Run with `--passWithNoTests` to exit with code 0
In D:\js\utility-types
  35 files checked.
  testMatch: D:/js/utility-types/src\?(*.)+(spec|test).ts?(x) - 0 matches
  testPathIgnorePatterns: \\node_modules\\ - 35 matches
  testRegex:  - 0 matches
Pattern:  - 0 matches
npm ERR! code ELIFECYCLE
npm ERR! errno 1
npm ERR! utility-types@3.9.0 test:update: `jest --config jest.config.json --no-cache -u && dts-jest-remap ./src/*.spec.ts --rename {{basename}}.snap.{{extname}}`
npm ERR! Exit status 1
npm ERR!
npm ERR! Failed at the utility-types@3.9.0 test:update script.

[captain-yossarian]

@piotrwitek I'm not sure it is possible to add second generic argument to $ObjMap as you did it in your example
How to asure TS that MapFn argument has generic parameter?

[piotrwitek]

It seems that the problem for tests on windows is testMatch path regex, I cannot help with that unfortunately.

[piotrwitek]

It might not be currently possible, but it's the goal for this type. Otherwise, it'll be a completely different type.

[captain-yossarian]

SO question related to this issue.