错误 TS2749：“IRequestWithUser”指的是一个值，但在此处用作类型。 您的意思是“typeof IRequestWithUser”吗？

Question

I am extending Request in express library to contain a user property:

import { Request } from 'express';

export default interface RequestWithUser extends Request {
  user: {
    user_name: string;
    password: string;
  }
}

The title error appears in the first parameter annotation:

import IRequestWithUser from '../shared/interfaces/isRequestWithUser';

const router: Router = Router();

router.post('/myRoute', async (req: IRequestWithUser, res: Response) => {

/*
error TS2749: 'IRequestWithUser' refers to a value, 
but is being used as a type here. Did you mean 
'typeof IRequestWithUser'?
*/

I don't believe interfaces are values. They should purely be types. So what is causing this error?

Also tried

typeof IRequestWithUser This results in No overload matches this call

Answer 1

Perhaps there's something missing from your code snippets, but the only error I get on Typescript 3.9 is one that relates to the overload error that you allude to later in your question.

Because the .post() call is looking for a callback that expects type Request , you can't type your req argument as IRequestWithUser . This is because, although IRequestWithUser will contain all properties of Request , the Typescript compiler can't guarantee that the extra properties you plan to have in req will be there when the callback is run.

The recommended way to extend Express types is to use interface merging . This allows you to "redefine" the Request type altogether, but it's a global definition so it gets messy:

// definitions/express/index.d.ts
import * as express from 'express' // Unfortunately, you need some kind of import here to make this a valid module.

declare module "express-serve-static-core" {
    export interface Request {
        user: {
            user_name: string;
            password: string;
        }
    }
}

Your compiler should pick this file up on its own and support your new property using the original Request type. You might need to have an explicit typeRoots set in your tsconfig.json if you're using something like ts-node though.