Problem with TypeScript conditional types and generics.

problem-with-ts-conditional-types-and-generics.ts

type Task<T> = () => T | Promise<T>;
type Mapper<T, R> = (instance: LoadingInstance<T>) => R;

interface LoadingInstance<T> {
  result: T | undefined;
  loading: boolean;
  loadingError: Error | null;
  reload: () => void;
}

/* FIRST SCENARIO - USING OVERLOADS */

// first overload - no mapper
function useLoading<T>(
  task: Task<T>,
  dependencies: any[],
  loadingDelay?: number
): LoadingInstance<T>;

// second overload - with mapper
function useLoading<T, R>(
  task: Task<T>,
  dependencies: any[],
  loadingDelay: number | undefined,
  mapper: Mapper<T, R>
): R;

// implementation
function useLoading<T, R>(
  task: Task<T>,
  dependencies: any[] = [],
  loadingDelay = 500,
  mapper?: Mapper<T, R>
): LoadingInstance<T> | R {
    throw new Error('NOT IMPLEMENTED');
}

/* Preparing some data for testing the typings */

const doTask = () => ({ listen: 'song' });

type DoTaskResult = ReturnType<typeof doTask>;

function extractListen(instance: LoadingInstance<DoTaskResult>) {
    const { result } = instance;

    return result && result.listen;
}
                               
/* First test - OK */

// usage 1 - expected result: LoadingInstance<DoTaskResult> (OK)
const result1 = useLoading(doTask, []);

// usage 2 - expected result: string | undefined (OK)
const listen1 = useLoading(doTask, [], undefined, extractListen);


/* SECOND SCENARIO - USING CONDITIONAL TYPES */

type ExtractLoadingInstance<T, TMapper> =
    TMapper extends Mapper<T, infer R> ? R
    : LoadingInstance<T>;

function useLoading2<T, TMapper extends Mapper<T, any>>(
  task: Task<T>,
  dependencies: any[] = [],
  loadingDelay = 500,
  mapper?: TMapper
): ExtractLoadingInstance<T, TMapper> {
    throw new Error('NOT IMPLEMENTED');
}

// usage 1 - expected result: LoadingInstance<DoTaskResult> (FAIL - result is any)
const result2 = useLoading2(doTask, []);

// usage 2 - expected result: string | undefined (OK)
const listen2 = useLoading2(doTask, [], undefined, extractListen);