_app.js

static async getInitialProps ({ Component, router, ctx }) {
  let page = {}

  if (Component.getInitialProps) {
    page = await Component.getInitialProps(ctx)
  }

  const basepath = typeof window === 'undefined' 
    ? '' 
    : location.pathname.slice(0, location.pathname.length - router.route.length).replace(/\/$/, '')

  return { page, basepath }
}