[Typescript] Extract the Result From Several Possible Function Shapes

Let's imagine you're building a type helper to extract out the value from several different 'parsers'.

const parser1 = {
  parse: () => 1,
};

const parser2 = () => "123";

const parser3 = {
  extract: () => true,
};

 

Expect to be:

type tests = [
  Expect<Equal<GetParserResult<typeof parser1>, number>>,
  Expect<Equal<GetParserResult<typeof parser2>, string>>,
  Expect<Equal<GetParserResult<typeof parser3>, boolean>>,
];

 

My solution to it:

type GetParserResult<T> = T extends {
  parse: () => infer TResult;
}
  ? TResult
  : T extends () => infer TResult
  ? TResult
  : T extends {
      extract: () => infer TResult;
    }
  ? TResult
  : never;

 

It is also possible using union + infer

type GetParserResult<T> = T extends
  | {
      parse: () => infer TResult;
    }
  | {
      extract: () => infer TResult;
    }
  | (() => infer TResult)
  ? TResult
  : never;