documentation/.vuepress/enhanceApp.js

export default ({
  Vue, // the version of Vue being used in the VuePress app
  options, // the options for the root Vue instance
  router, // the router instance for the app
  siteData, // site metadata
  isServer // is this enhancement applied in server-rendering or client
}) => {
  findVersionedPaths(siteData.themeConfig.sidebar).forEach(vp => {
    router.addRoutes(
      router.options.routes.map(route => {
        if (route.path.startsWith(vp.path + vp.currentVersion)) {
          return [
            {
              path: route.path.replace(vp.currentVersion, "current"),
              redirect: route.path
            }, {
              path: route.path.replace(vp.currentVersion + "/", ""),
              redirect: route.path
            }
          ]
        }
        return undefined
      }).filter(x => x).flat()
    )
  })
}

function findVersionedPaths(paths) {
  return Object.entries(paths).map(([path, children]) => {
    return children
      .filter(child => Array.isArray(child.versions))
      .map(child => ({ ...child, path: pathJoin(path, child.path) }))
  }).flat()
}

// https://stackoverflow.com/a/29855282/4430124
function pathJoin(...parts) {
  var separator = '/';
  var replace = new RegExp(separator + '{1,}', 'g');
  return parts.join(separator).replace(replace, separator);
}