Preserve comments for mapped types in d.ts

Question

In the latest Typescript (v5.0.4 as for today) when I compile declarations for the following code:

type SomeType<T> = {
  field: T;
};

const getInstance = <T>(
  value: T
): SomeType<{
  [K in keyof T]: T[K];
}> => {
  return {
    field: value,
  };
};

export const variable = getInstance({
  /**
   * want to have it in d.ts
   */
  nested: "nested",
});

I'm getting this in my d.ts:

type SomeType<T> = {
    field: T;
};
export declare const variable: SomeType<{
    nested: string;
}>;
export {};

Which reflects what we have in the code, but unfortunately there is no comment. I happens specifically when I use mapped types in the returned type.

It would have been work fine If I don't use mapped type, but unfortunately I can't get rid of it in my case.

The question is how to preserve the comment in the d.ts file with mapped type? Nice stuff is that I see my comment in VSCode when I hover over variable.field.nested property which means that Typescript preserves it somewhere but just doesn't add it to d.ts file on declaration generation stage.

Answer 1

I can't find authoritative documentation for exactly how the compiler chooses to preserve or discard comments in declaration files, but since you said it works fine if you just wrote SomeType<T> instead of SomeType<{[K in keyof T]: T[K]}>, that suggests comments might be preserved when using a generic type directly like XXX<T>. So what if we defined an XXX that behaves how you want?

// .ts file
type GetInstance<T> =
    SomeType<{ [K in keyof T]: T[K] }>;

const getInstance = <T,>(value: T
): GetInstance<T> => {
    return {
        field: value,
    };
};

export const variable = getInstance({
    /**
     * want to have it in d.ts
     */
    nested: "nested",
});

This is equivalent to your code. And when we check the generated declaration file, the comment is indeed preserved!

// .d.ts file
export declare const variable: GetInstance<{
    /**
     * want to have it in d.ts
     */
    nested: string;
}>;
export {};

If anyone finds where such behavior is documented I'd be happy to add it here.

Playground link to code