It seems you are using the no-misused-promises rule which states that you cannot return Promise<void> in a place where void is expected.

This means that you cannot return Promise<void> from your Express handler because the return type of RequestHandler from the library specifies that the return type should be void. I would suggest that you change it to return Promise<Response> by adding a simple return keyword:

import { getConnection } from 'typeorm';
import express from 'express';
const router = express.Router();

const resetDatabase = async (): Promise<void> => {
  const connection = getConnection();
  await connection.dropDatabase();
  await connection.synchronize();
};

// typescript-eslint throws an error in the following route:
router.post('/reset', async (_request, response) => {
  await resetTestDatabase();
  return response.status(204).send();  // <----- return added here
});

export default router;

The other option would be to avoid using async/await:

router.post('/reset', (_request, response) => {
  resetDatabase().then(() => response.status(204).send());
});

I found a solution that doesn't involves using then() and let you use the abstraction of async without getting cursed by the eslint, there's two solutions (but i recommend more the second one)

First Solution: Using "inside async"

This is basic using a async inside the void like this:

router.post('/reset', (_request, response) => {
    (async () => {
        await resetTestDatabase();
        response.status(204).end();
    })()
});

Second Solution (recommended): "Type Overlap"

The second option is to you use it as an async, as Always, but say "hey TypeScript, nothing is wrong here hehe" with the "as" keyword

import { RequestHandler } from 'express'

router.post('/reset', (async (_request, response) => {
    await resetTestDatabase();
    response.status(204).end();
}) as RequestHandler);