The TypeScript Generics issue arises when attempting to assign the type '{ result: true }' to type 'T'

Struggling with implementing generic types, I can't seem to make it work properly. It feels like I'm missing something simple. Here's a basic example:

// some module
type TMainResponse<T> = {
  data: T;
};

interface Foo {
  func<T>(): Promise<TMainResponse<T>>;
}

// local module
type TLocalResponse = {
  result: boolean;
};

const obj: Foo = {
  async func<TLocalResponse>() {
    return {
      data: {
        result: true
      }
    };
  }
};

(async function () {
  await obj.func();
});

The outcome is as follows:

Type 'Promise<{ data: { result: boolean; }; }>' cannot be assigned to type 'Promise<TMainResponse<T>>'.
    Type '{ data: { result: boolean; }; }' is not compatible with type 'TMainResponse<T>'.
      The types of the 'data' property are incongruent.
        Type '{ result: boolean; }' cannot be assigned to type 'T'.

16   async func<TLocalResponse>() {
           ~~~~

  src/test.ts:7:3
    7   func<T>(): Promise<TMainResponse<T>>;
        ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    The expected type is specified by the 'func' property declared in the 'Foo' interface

I am wondering if there is an error on my end, or if I have simply misunderstood how generics should be used?

Answer №1

Could I be making a mistake here, or am I simply misinterpreting the use of generics?

This particular interface seems somewhat unconventional:

interface Foo {
  func<T>(): Promise<TMainResponse<T>>;
}

The Foo does not have a type argument, yet <T> is utilized in two instances within its body. Frankly, it's unclear to me how that should function as intended (given that there are no compilation errors, it likely operates by design -- any insights on this would be appreciated). However, if you modify the interface to include the type argument:

interface Foo<T> {
  func(): Promise<TMainResponse<T>>;
}

And adjust the usage accordingly:

const obj: Foo<TLocalResponse> = {
  async func() {
    return {
      data: {
        result: true
      }
    };
  }
};

Then the entire system functions smoothly.

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