hideNav = (navbar) => {
    const hide = () => {
      let lastScrollTop = 0;
      const currentScrollTop = navbar.scrollTop;

      // scroll down
      if (currentScrollTop > lastScrollTop) {
      navbar.classList.add('hidden');
      } else {
      // scroll up
        navbar.classList.remove('hidden');
      }
      lastScrollTop = currentScrollTop;
    };

    window.addEventListener('scroll', hide);
  };

render() {
      return <Navbar ref={this.hideNav} />