Implement authentication for Next.js with external backend (JWT)

Introduction Usually when you build a new project from scratch with Next.js you don't create a separate backend because it adds more complexity and slow you down, especially for MVPs! We mostly go with solutions that don't let us worry or deeply invest in authentication like third party services such as Clerk , a BaaS like Supabase or even Better Auth , so we have auth ready in seconds and we can ship fast. But what about when we are required to work with an existing backend, no matter what tech