What are feature flags in a MERN SaaS?

Feature flags are Boolean switches controlling which features are visible to which tenants or tiers, without a code deployment. We implement them in Express middleware, evaluated before any handler runs. Flag configurations are stored in MongoDB with a Redis cache for sub-millisecond evaluation at the request level. Use cases: gradual rollouts to 10% of tenants, per-tier feature gating (Basic vs Pro vs Enterprise), A/B tests across tenant cohorts, kill switches for unstable features, and per-tenant enterprise overrides for negotiated custom plans.