Canonical URL là gì? Khi nào cần sử dụng?

Canonical URL (<link rel='canonical' href='...'>) cho Google biết URL nào là 'bản gốc' khi có nhiều URLs chứa cùng nội dung.

Ví dụ: /products?sort=price và /products?sort=name cùng nội dung → canonical trỏ về /products. Khi nào CẦN:

1. URL parameters tạo duplicate (filters, sort, pagination)
2. HTTP vs HTTPS, www vs non-www
3. Syndicated content (bài viết đăng trên nhiều site)
4. Mobile URL khác desktop URL. Nếu không có canonical, Google tự chọn 'bản gốc' — có thể chọn sai. Trong Next.js: dùng metadata.alternates.canonical trong generateMetadata()

Lưu ý: canonical phải là absolute URL, self-referencing canonical cũng tốt (mỗi trang canonical trỏ về chính nó).

A canonical URL (<link rel='canonical' href='...'>) tells Google which URL is the 'original' version when multiple URLs share the same content. For example: /products?sort=price and /products?sort=name show the same content — the canonical should point to /products. When you NEED it:

1. URL parameters creating duplicates (filters, sorting, pagination)
2. HTTP vs HTTPS or www vs non-www versions
3. Syndicated content (articles published on multiple sites)
4. Mobile URLs that differ from desktop URLs. Without a canonical, Google picks its own 'original' — which may be the wrong choice. In Next.js: use metadata.alternates.canonical inside generateMetadata()

Note: canonical URLs must be absolute, and self-referencing canonicals (each page pointing to itself) are also recommended.