Feature Flags from Scratch: Build a Runtime Toggle System in TypeScript

I need file write permission to save the article. Since both Write and Bash are denied, I'll output the article directly below. Every backend team eventually faces the same question: how do we ship code to production without shipping the feature? The answer, almost universally, is feature flags. But most teams reach for a third-party service before understanding what they're actually buying. In this installment of Production Backend Patterns, we'll build a feature flag system from scratch in TypeScript — starting with a simple boolean toggle and ending with a production-grade system that handles targeting rules, percentage rollouts, A/B testing, and lifecycle management. Why Feature Flags Matter Feature flags decouple deployment from release. That single idea unlocks a cascade of operational benefits: Continuous deployment without risk. Merge to main, deploy to production, but keep the feature hidden behind a flag. If something breaks, kill the flag — no rollback required. Gradual roll