A higher-order function that takes a route handler function and returns another route handler that parses JSON bodies before invoking the inner handler:

interface PersonRequestBody {
  name: string;
}

interface NameLengthResponseBody {
  length: number;
}

const getNameLength = withJsonBody<PersonRequestBody>(({ parsedBody }) =>
  jsonResponse<NameLengthResponseBody>({
    length: parsedBody.name.length,
  })
);