apiMethodWrapper.ts

// First we define a standard generic structure for an api response
interface ApiMethodResponse<T> {
  statusCode?: number
  data: T
}

// The `apiMethod` constructor will take an `ApiMethodDefinition` as argument.
type ApiMethodDefinition<T> = (req: Request) => Promise<ApiMethodResponse<T>>

// Now the constructor:
export function apiMethod<T> (f: ApiMethodDefinition<T>): RequestHandler {
  // here we take benefit from Request & Response typings
  return async function (req: Request, res: Response) {
    // No way you forget the try/catch
    try {
      // call the apiMethod definition
      const {statusCode, data} = await f(req)
      if (data) {
        // default the status to 200, and send data if any
        res.status(statusCode || 200).send(data)
      } else {
        // default the status to 200 and send empty response
        res.sendStatus(statusCode || 200)
      }
    } catch (e) {
      // always catch any unexpected error
      res.status(500).send(e)
    }
  }
}

I found that you have to put a question mark also on data prop in the ApiMethodResponse interface, because you return also an object without generic T if db returns null if (!user) {return {statusCode: 404}}

interface ApiMethodResponse<T> {
  statusCode?: number
  data?: T
}