useRouter() return undefined

useRouter must be called inside of setup() (documentation link)

But you can use composition api for extract logic of routing

<script>
import { useRouter } from "vue-router"

const useRouterCustom = () => {
  const router = useRouter();

  const goToRoot = () => {
    router.push('/')
  }
  return {
    goToRoot
  }
}

export default {
  name: 'App',
  setup() {
    const { goToRoot } = useRouterCustom();
  }
  
}
</script>

I hope it helps, good luck!

That composables should only be used in Vue components (in setup() is now documented here:

https://vuejs.org/guide/reusability/composables.html#usage-restrictions