Issue with tsconfig in strict mode and Type (...) is not assignable to type (...)

Question

I know there are quite a lot of question about this, I did read most of them (I guess) and they helped me having a better understanding of this error, and how to solve it in most cases.

However I am facing one I could not resolves by myself, so apologies if SO is not the good place for this question (I guess this could be a dupe since there is already questions about this issue, but I did not found any that helped me to solve this "particular case").

I have the following piece of code :

import { Request } from 'express'
import multer from 'multer'
import path from 'path'

class ImageMiddleware {

    // ...

    private storage (folder: string): multer.StorageEngine {
        return multer.diskStorage({
            destination: (
                _req: Request,
                _file: Express.Multer.File,
                callback: (error: Error | null, destination: string) => void
            ): void => {
                return callback(null, `./public/img/${folder}`)
            },
            filename: (
                req: Request,
                file: Express.Multer.File,
                callback: (error: Error | null, destination: string) => void): void => {
                callback(
                    null,
                    `${file.fieldname}`
                    + `-`
                    + `${Date.now()}`
                    + `${req.user.id}`
                    + `${path.extname(file.originalname)}`
                )
            },
        })
    }

    // ...
}

export default new ImageMiddleware()

With this tsconfig.json :

{
    "compilerOptions": {
        "target": "ES6",
        "module": "commonjs",
        "outDir": "./dist/",
        "sourceMap": true,
        "removeComments": true,
        "pretty": true,
        "noUnusedLocals": true,
        "noUnusedParameters": true,
        "alwaysStrict": true,
        "noImplicitThis": true,
        "noImplicitAny": true,
        "allowUnreachableCode": false,
        "esModuleInterop": true,
        "strict": true
    },
    "exclude": ["**/spec/*.spec.ts", "./node_modules"]
}

But when running ./node_modules/typescript/bin/tsc I have the following error :

services/middlewares/image/image.middleware.ts:112:13 - error TS2322: Type '(_req: Request, _file: File, callback: (error: Error | null, destination: string) => void) => void' is not assignable to type 'string | ((req: Request, file: File, callback: (error: Error | null, destination: string) => void) => void) | undefined'.
  Type '(_req: Request, _file: File, callback: (error: Error | null, destination: string) => void) => void' is not assignable to type '(req: Request, file: File, callback: (error: Error | null, destination: string) => void) => void'.
    Types of parameters '_req' and 'req' are incompatible.
      Type 'Request' is missing the following properties from type 'Request': get, header, accepts, acceptsCharsets, and 71 more.

112             destination: (
                ~~~~~~~~~~~

  node_modules/@types/multer/index.d.ts:59:9
    59         destination?: string | ((req: Express.Request, file: Express.Multer.File, callback: (error: Error | null, destination: string) => void) => void);
               ~~~~~~~~~~~
    The expected type comes from property 'destination' which is declared here on type 'DiskStorageOptions'

So I guess that the ): void => { line is wrong, hence I tried a lot of stuff to debug it but did not find anything successfull.

The output of ./node_modules/typescript/bin/tsc -v is Version 3.3.4000

Any help would be welcome, please let me know if I forgot some relevant informations

Answer 1

Because this error is in node_modules the only hope of fixing it is a pull request to the typings repository.

For now to silence the error turn "skipLibCheck": true in tsconfig.

EDIT: Sorry misread where error was coming from.

"Types of parameters '_req' and 'req' are incompatible." I Suspect if you're using express you've ended up with two types for "Request" there is one in the standard typescript library which 1 function is typed with and another type for "Request" that exists in express that express uses.