Can I override any type property in Typescript?

Question

I'm trying to attach my custom types into Express's Request/Response interface.

But some properties are of type any, so I'm having really hard to override it.

ex:

// @types/expres-server-static-core
interface Request {
  cookies?: any;
}

// my.ts
import type {Request} from 'express';

type MyCookies = {
  'apple'?: string;
  'banana'?: string;
}

interface MyRequest extends Request {
  cookies: MyCookies
}

// my-usecase.ts
const request: MyRequest = /* */; // MyRequest | Request
request.cookies; // still has any type because MyCookies | any will be any type

I know anytype is the top type of typescript so customized cookiesproperty will be ignored.
So I'm using like:

const myRequest: MyCookies = req.cookies;
// or
const myRequest = req.cookies as MyCookies;

It can solve this problem but I don't want to declare MyCookies constants; req.cookies = MyCookies is easier to use.

Is there any good way to override any type property?

Answer 1

Do not override default properties like that, because at "runtime" there is no guarantee req.cookies will exist. So to get correct type, do type narrowing.

const handler = (req: Request, res: Response) => {
  if (!req.cookies) throw new Error('unauthorised')
  if (!req.cookies['cookie_name_1'] || typeof req.cookies['cookie_name_1'] !== 'string') throw new Error('unauthorised')
  const cookie_name_1 = req.cookies['cookie_name_1'] // cookie_name_1 is string
}

Above one ensures existence of req.cookies at both runtime & compilation.

For auto-completion

type MyCookies = {
  auth_token?: unknown
  some_token?: unknown
}

const handler = (req: Request, res: Response) => {
  if (!req.cookies) throw new Error('unauthorised')
  const cookies = req.cookies as MyCookies 
  if (typeof cookies.auth_token !== 'string') throw new Error('unauthorised')
  console.log(cookies.auth_token) // type is string & IDE will provide autocompletion
}

You could override Request['cookies'] to to have MyCookies type but there is huge chance you access those cookie in one place, aka inside a middleware function.