Basic mobile menu functionality with tab cycle locking

mobile-menu.js

import $ from 'jquery';

export default function() {
  const $body = $('body');
  const $hamburger = $('.js-header-hamburger');
  const $mobileMenu = $('.js-mobile-menu');
  const $lastMenuItem = $mobileMenu.find('a').last();
  const $close = $('.js-mobile-menu-close');
  const $content = $('.js-mobile-menu-content');

  $hamburger.click(() => {
    $body.addClass('has-mobile-menu'); // body -> overflow: hidden;
    setTimeout(() => {
      $content.addClass('is-visible');
      $close.focus();
    }, 0);
  });

  $close.click(() => {
    $body.removeClass('has-mobile-menu');
    $content.removeClass('is-visible');
    $hamburger.focus();
  });

  $mobileMenu.on('keydown', 'a, button', function lockTabCycle({
    which,
    shiftKey,
  }) {
    if (which !== 9) {
      return true;
    }

    const $this = $(this);

    if (shiftKey && $this.is($close)) {
      $lastMenuItem.focus();
      return false;
    }

    if (!shiftKey && $this.is($lastMenuItem)) {
      $close.focus();
      return false;
    }

    return true;
  });
}