This was originally published on rollgate.io/blog/feature-flags-nodejs . Why Feature Flags in Node.js? Node.js powers a huge slice of production backends — REST APIs, GraphQL gateways, background workers, BFF layers, real-time services. All of them share the same release problem: you want to ship code continuously, but you do not want every deploy to be a product change. Feature flags in Node.js decouple deployment from release. You push code to production behind a flag and decide later who sees the new behavior, when, and under what conditions. If something breaks, you flip the flag off in the dashboard — no redeploy, no rollback PR, no pager at 3am. This guide covers the practical side: how to wire feature flags into Express and Fastify applications, how to target specific users, how to roll out gradually, and the production gotchas that every team hits sooner or later. Quick Start: Feature Flags in Node.js Let us get a flag running end-to-end.…