Sure, here's an example of a basic express app that uses the express router.

**main.js**

```js
const express = require('express')
const app = express()

// routes
app.use('/', require('./routes/index.js'))

// fallback route
app.use((req, res, next) => {
    res.status(404).send('404: Page not found')
})

// server
app.listen(3000, () => console.log('App running on port 3000'))
```
**./routes/index.js**
```js
const express = require('express')
const router = express.Router()

router.get('/', async (req, res) => {
    res.send('Hello World!')
})

module.exports = router
```
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