How to fix the “Typescript Error: Property ‘user’ does not exist on type ‘Request'” error with Express?

To fix the "Typescript Error: Property ‘user’ does not exist on type ‘Request’" error with Express, we can add our own definition for the request type.

For instance, we add

IGetUserAuthInfoRequest.ts

import { Request } from "express";
export interface IGetUserAuthInfoRequest extends Request {
  user: string;
}

into a TypeScript type definition file.

We create the IGetUserAuthInfoRequest interface the extends the Request interface by adding the user field.

Then we use it by writing

import { Response } from "express";
import { IGetUserAuthInfoRequest } from "./IGetUserAuthInfoRequest";

app.get(
  "/auth/userInfo",
  validateUser,
  (req: IGetUserAuthInfoRequest, res: Response) => {
    res.status(200).json(req.user);
  }
);

to import IGetUserAuthInfoRequest and use it as the type for the req parameter instead of Request.