Passing generic variables to nested functions

Question

I'm trying to write type definitions for an "async-route" wrapper for express.js. I'm new to typescript, just learned about generics.

As the function could be called behind middlewares that extend the default express Request I want to create it so users can provide their own extended "Request".

So far I have gotten to:

asyncRoute.ts

import { NextFunction } from 'express';

type Fn = <R, S>(req: R, res: S, next?: NextFunction) => Promise<any>;

export default function asyncRoute<R, S> (fn: Fn) {
  return function asyncRouteWrap(req: R, res: S, next: NextFunction) {
    Promise.resolve(fn<R,S>(req, res, next)).catch(next);
  };
};

Usage:

import { Request } from 'express';

interface AppRequest extends Request {
  log: Logger;
  // ...other stuff
}

router.get(
  '/',
  asyncRoute<AppRequest, AppResponse>(async (req, res) => {
    req.log.info('test');
  })
);

But I get Property 'log' does not exist on type 'Request'.

FK82 · Accepted Answer · 2018-12-28 07:35:52Z

The problem is that the TypeScript compiler cannot infer the type of req in the anonymous function parameter of asyncRouter. If you have IDE support for TypeScript you will see that fn<R, S>(req, res, next) does not have R bound to AppRequest (in VS Code you only need to hover over the specific line of code).

You can solve your problem by explicitely adding type parameters to Fn in your definition of asyncRoute. For this, you need to first add type parameters to your type definition for Fn

type Fn<R, S> = (req: R, res: S, next?: NextFunction) => Promise<any>

and then define asyncRoute as follows

export default function asyncRoute<R, S> (fn: Fn<R, S>) { 
  return function asyncRouteWrap(req: R, res: S, next: NextFunction) {
    Promise.resolve(fn(req, res, next)).catch(next);
  };
};

The type error should be gone now.

Here's a Typescript Playground with the full code. This contains a few mocked interfaces/types but I'm sure you'll get the point.

Collectives™ on Stack Overflow

Passing generic variables to nested functions

1 Answer 1

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

Comments

Your Answer

Sign up or log in

Post as a guest

Related