Loader.ts

type LoaderProps<T> = {
  fetching: boolean
  data: T
}

const Loader: FC<LoaderProps> = ({ fetching, data, children }) => (
  if (fetching) {
    return <div>Loading...</div>
  }
  return children(data)
)

const App = () => (
  // Data maybe undefined
  <Loader fetching={true} data={data}>
    // Data should be defined
    {data => <div>{data}</div>}
  </Loader>
)