External callback function

_The following examples are based (but no dependent) on [Express router](https://expressjs.com/en/guide/routing.html)._

**Considering the following correct case:**

[8.1 When you must use function expressions (as when passing an anonymous function), use arrow function notation. ](https://github.com/airbnb/javascript#arrows--use-them)

```
router.get('/url', (req, res) => {
...
});
```

**Should you?**

Write the callback function as an arrow function as well, if called from an external place?

```
// controllers.js
export const getUrlController = (req, res) => {
...
});
```

```
// routes.js
import { getUrlController } from './controllers'

router.get('/url', urlController);
```

Or should it rather be a function in this case?

```
// controllers.js
export const getUrlController = function getUrl(req, res) {
...
};
```

Start by reviewing the linked Airbnb arrow-function guidance alongside the Express router examples in the issue. Compare the controllers.js and routes.js examples, then look for a maintainer decision on whether externally defined callbacks need a specific style rule; the issue is done when that guidance is clearly documented.

Written by the indexing model from the issue text.