What is the correct way to write an asynchronous Express middleware function in Typescript?

Can anyone help me figure out how to correctly define a return value for an express middleware that utilizes async/await? I've been experimenting with different approaches but haven't found success yet.

Additionally, I'm attempting to extend the Request object in order to set req.user after decoding the JWT payload.

import { NextFunction, Request, RequestHandler, Response } from 'express'

const authenticate: RequestHandler = async (req: IUserRequest, res: Response, next: NextFunction): Promise<void> => {
  if (! req.headers.authorization) { return next(new ValidationError()) }
  const payload: IUserData = await decodeJwt(req.headers.authorization)
  req.user = payload
  return next()
}

export interface IUserRequest extends Request {
  user: IUserData
}

/* errors 
(req: IUserRequest, res: Response, next: NextFunction) => Promise<void>' is not assignable to type 'RequestHandler'.
  Types of parameters 'req' and 'req' are incompatible.
    Type 'Request' is not assignable to type 'IUserRequest'.
      Property 'user' is missing in type 'Request'.
*/

Answer №1

Request and IUserRequest types do not match, as the user property is missing in the Request type. This property may not exist in the req object until it is assigned to it.

The corrected version should be:

export interface IUserRequest extends Request {
  user?: IUserData
}

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