Mixins with TypeScript?

Since starting this thread I have found this thread:

I’m creating a mixin which a bunch of components will use for nested routes, to “promote” router parameters to top-level properties.

TypeScript complains if I try to create the following structure:

export const RouteHelper = {

  computed: {
    siteId () {
      // TS2339: Property '$route' does not exist on type '{ siteId(): any; companyId(): any; appId(): any; }'.
      return this.$route.params.siteId
    },
    ...

With a bit of playing around I’ve managed to hack it like this, but not sure if it is the best way?

export const RouteHelper: ComponentOptions<any> = {

  computed: {
    siteId () {
      return (this.$route as Route).params.siteId
    },
    ...

How should this best be done?