Create dynamic function signature

dynamic-fn-signature.ts

type Person =
  | {
      firstName: string;
      lastName: string;
      isMarried: false;
    }
  | {
      isMarried: true;
      spouse: {
        spouseFirstName: string;
        spouseLastName: string;
      };
    };

export function createPerson<T extends Person["isMarried"]>(
  isMarried: T,
  ...args: Extract<Person, { isMarried: T }> extends { spouse: infer S }
    ? [spouse: S]
    : never
) {
  console.log(args);
}

// Passing `true` to isMarried, requires you to also supply spouse info.
// This function call errors out because spouse object is missing.
createPerson(true)

// This function call works with spouse object
createPerson(true, { spouseFirstName: "John", spouseLastName: "Doe" });

// Passing `false` to isMarried does not require spouse object.
// This function call works without passing in the spouse object.
createPerson(false);

/** Closure style 

export function createPerson<T extends Person["isMarried"]>(isMarried: T) {
  return (
    ...args: Extract<Person, { isMarried: T }> extends { spouse: infer S }
      ? [spouse: S]
      : never
  ) => {
    console.log(args);
  };
}

*/