The _next/data route is a fundamental aspect of the Next.js framework, specifically designed to handle Client-side-Navigation efficiently. When a user navigates using the Link-component, the application fetches a JSON file containing the pre-calculated props for the target page instead of requesting a full HTML document. This file is located at a path following the pattern /_next/data/[buildID]/[page].json, which is essential for the Hydration process on the client.
This mechanism supports both Static-Generation and Server-side-Rendering. For pages utilizing getStaticProps, the JSON data is generated at build time. Conversely, for pages using getServerSideProps, the data is fetched on the server at request time and returned as a JSON payload. Detailed specifications can be found in the Next.js Data Fetching Documentation and the Vercel Framework Guide.
A key component of this system is the Build-ID, a unique hash that identifies the specific deployment of the application. The Router uses this ID to ensure that the JavaScript logic on the client matches the data structure provided by the server. If a mismatch occurs, typically during a new deployment on Vercel, the system can trigger a full page reload to maintain application integrity and Web-Performance.