How to use arr-union

topics.get(owner, repo, options)
  .then(function(res) {
   if (res.message && /invalid/i.test(res.message)) {
    return Promise.resolve(res);
   }

   var opts = topics.normalize(owner, repo, options);
   opts.names = union([], res.names, opts.names);
   return topics.put(opts);
  })

 union(matches, files);
});