index.js vs page.js Belong to Two Mutually Exclusive Next.js Routing Systems

index.js and page.js are interchangeable within the same routing system, and Next.js checks for both at build time

page.js is the modern replacement and index.js will throw a build error in any Next.js 13 or later project

They belong to two separate, mutually exclusive routing systems - index.js marks a folder's root route in the Pages Router, while page.js marks a route segment in the newer App Router - and mixing the convention from one system into the other's directory has no effect

index.js is used for dynamic routes and page.js is used exclusively for static routes within the same router