How can I use generics in a function returned from a factory function that is typed?

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I have defined a type for a factory function where one of the functions returned uses a generic.

type Factory = (param: string) => {
  func: <T extends {}>(param: string) => Promise<T>,
};

const factory: Factory = __factoryParam => ({
  func: async <T>(__param) => {
    return Promise.resolve({} as T);
  },
});

When I hover over the __param parameter in the code above I get the error Parameter '__param' implicitly has an 'any' type.ts(7006).

If I get rid of the generic, then __param is typed correctly. If I explicitly type the parameter in the implementation, it of course works:

func: async <T>(__param: string) => {

What am I doing wrong here?

typescript generics
1 Answers

This is a case where it's fine to use as any on the return value and just omit the generic in the implementation:

TS Playground

type Factory = (param: string) => {
  func: <T extends {}>(param: string) => Promise<T>,
};

const factory: Factory = __factoryParam => ({
  async func (__param) {
    return Promise.resolve({}) as any;
  },
});

const result = factory('str').func<{msg: 'hello'}>('str2'); // Promise<{ msg: 'hello' }>
user_4382730
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