I'm trying to create a strongly typed function in TypeScript that infers the type of params.srcData based on the src function inside the same object.

Here’s what I have so far:

import { Observable } from 'rxjs';

type GetSrc<T> = T extends { src: () => Observable<infer TSrc> } ? TSrc : never;

function myFunction4<
  TData,
  T extends {
    [key in keyof T]: {
      src: () => Observable<unknown>;
    } extends any
        ? {
            src: () => Observable<GetSrc<T[key]>>;
            api: (params: { srcData: GetSrc<T[key]> }) => Observable<TData>;
          }
      : never;
  }
>(obj: T) {
  return obj;
}

const result4 = myFunction4({
  test1: {
    src: () => new Observable<number>(),
    api: (params) => { 
      // I would like params.srcData to be inferred as "number"
      const srcData = params.srcData; 
      return new Observable<number>();
    },
  },
  test2: {
    src: () => new Observable<string>(),
    api: (params) => { 
      // I would like params.srcData to be inferred as "string"
      const srcData = params.srcData; 
      return new Observable<number>();
    },
  },
} as const);

Expected Behavior:

params.srcData in test1.api should be inferred as number params.srcData in test2.api should be inferred as string

Actual Behavior:

params.srcData is inferred as never

Question:

How can I correctly infer the type of params.srcData from src in my function definition? What adjustments are needed in my TypeScript generic types?

Here a link where I reproduce the problem: ;file=index.ts

I'm trying to create a strongly typed function in TypeScript that infers the type of params.srcData based on the src function inside the same object.

Here’s what I have so far:

import { Observable } from 'rxjs';

type GetSrc<T> = T extends { src: () => Observable<infer TSrc> } ? TSrc : never;

function myFunction4<
  TData,
  T extends {
    [key in keyof T]: {
      src: () => Observable<unknown>;
    } extends any
        ? {
            src: () => Observable<GetSrc<T[key]>>;
            api: (params: { srcData: GetSrc<T[key]> }) => Observable<TData>;
          }
      : never;
  }
>(obj: T) {
  return obj;
}

const result4 = myFunction4({
  test1: {
    src: () => new Observable<number>(),
    api: (params) => { 
      // I would like params.srcData to be inferred as "number"
      const srcData = params.srcData; 
      return new Observable<number>();
    },
  },
  test2: {
    src: () => new Observable<string>(),
    api: (params) => { 
      // I would like params.srcData to be inferred as "string"
      const srcData = params.srcData; 
      return new Observable<number>();
    },
  },
} as const);

Expected Behavior:

params.srcData in test1.api should be inferred as number params.srcData in test2.api should be inferred as string

Actual Behavior:

params.srcData is inferred as never

Question:

How can I correctly infer the type of params.srcData from src in my function definition? What adjustments are needed in my TypeScript generic types?

Here a link where I reproduce the problem: https://stackblitz/edit/bskqm7z9?devtoolsheight=50&file=index.ts

• typescript

• If you need more engagement here, consider editing your minimal reproducible example to be pure TS without a dependency on rxjs or any external/third-party frameworks. – jcalz Commented Mar 14 at 22:10

1 Answer 1

Given high order functions, input and output infers and an argument object with different value types it's hard to provide solution with exact call signature you are using. But there's an alternative to use separate method calls to fill the object with a probably simpler approach of using a generic type for an api pair (you could give the class a better name):

Playground

import { Observable } from 'rxjs';

type ApiPair<T, R> = {
  src: () => Observable<T>;
  api: (params: { srcData: T }) => Observable<R>;
};

class Api{
  add<T, K extends string, R>(key: K, pair: ApiPair<T,R>): asserts this is this & {[k in K]: ApiPair<T, R>}{
    (this as any)[key] = pair;
  }
}

const api: Api = new Api;

api.add('test1', {
    src: () => new Observable<number>(),
    api: (params) => { 
      // I would like params.srcData to be inferred as "number"
      const srcData = params.srcData; 
      return new Observable<number>();
    },
});

api.add('test2',{
  src: () => new Observable<string>(),
  api: (params) => { 
    // I would like params.srcData to be inferred as "string"
    const srcData = params.srcData; 
    return new Observable<number>();
  },
});

api.test2 // (property) test2: ApiPair<string, number>