BaseHandler handler method return type should take into account async handlers

[islazh]

Is there an existing issue for this?

• I have searched the existing issues

Current behavior

When creating a middleware with a handler, the handler's type is

abstract handler(req: express.Request, res: express.Response, next: express.NextFunction): void;

Express middleware handlers may be async, so the return type should be void | Promise<void>

Steps to reproduce

Create an async middleware handler with no-misused-promises turned on in ESLint config

Expected behavior

No ESLint errors

Possible solution

ESLint warning:

Promise-returning method provided where a void return was expected by extended/implemented type 'BaseMiddleware'

Package version

6.0.1

Node.js version

No response

In which operating systems have you tested?

• macOS
• Windows
• Linux

Stack trace

No response

Other

No response

[notaphplover]

Hey @islazh, after having a look at the docs I don't think Express middleware handlers may be async. Having a look at express types:

export interface RequestHandler<
    P = ParamsDictionary,
    ResBody = any,
    ReqBody = any,
    ReqQuery = ParsedQs,
    LocalsObj extends Record<string, any> = Record<string, any>,
> {
    // tslint:disable-next-line callable-types (This is extended from and can't extend from a type alias in ts<2.2)
    (
        req: Request<P, ResBody, ReqBody, ReqQuery, LocalsObj>,
        res: Response<ResBody, LocalsObj>,
        next: NextFunction,
    ): void;
}

Closing the issue now. Feel free to provide a code example of an asyncronous middleware working on express and I'll reopen the issue.

EDIT: My bad, After having a look at the right types:

export interface RequestHandler<
    P = ParamsDictionary,
    ResBody = any,
    ReqBody = any,
    ReqQuery = ParsedQs,
    LocalsObj extends Record<string, any> = Record<string, any>,
> {
    // tslint:disable-next-line callable-types (This is extended from and can't extend from a type alias in ts<2.2)
    (
        req: Request<P, ResBody, ReqBody, ReqQuery, LocalsObj>,
        res: Response<ResBody, LocalsObj>,
        next: NextFunction,
    ): void | Promise<void>;
}

It seems it's perfectly fine to provide an async handler

[notaphplover]

Ok, I had a look at some articles and did some tests and it seems it's possible. Reopening the issue