const uniqueTags = posts.reduce((acc, post) => {
  acc.push(...post.tags);
  return [...new Set(acc)];
}, []);

const uniqueTags = posts.reduce((acc, post) => {
  return Array.from(new Set(acc.concat(post.tags)));
}, []);

const uniqueTags = posts.reduce((acc, post) => {
  post.tags.forEach((tag) => {
    if (!acc.includes(tag)) acc.push(tag);
  });
  return acc;
}, []);

const uniqueTags = posts.reduce((acc, post) => acc.add(post.tags), new Set());