Reading OpenSource lib that write in TypeScript is a nightmare

WTF:

export function concatMap<T, I, R>(
project: (value: T, index: number) => ObservableInput<I>,
resultSelector?: (outerValue: T, innerValue: I, outerIndex: number, innerIndex: number) => R
): OperatorFunction<T, I|R> {
return mergeMap(project, resultSelector, 1);
}

That is just fucking definition, no execution code inside