A practical guide to implementing feature flags in JS/TS: what they are, when to use them, and how to avoid the traps that make them painful to manage. Feature flags are a conditional: if (flagEnabled) { show new thing } else { show old thing }. That’s the entire concept. The value comes from where the condition is evaluated and who controls it — not the developer at deploy time, but a dashboard at runtime. This guide covers the core patterns, when each one applies, and what separates a well-managed flag from one that quietly rots in your codebase. The basic pattern A feature flag SDK gives you a function. You call it with a flag key, optionally a user context, and it returns a boolean (or a variant, for multivariate flags). import { flaggy } from ' @flaggy.io/sdk-js ' ; const client = flaggy ({ apiKey : process . env . FLAGGY_API_KEY }); await client . initialize (); if ( client . isEnabled ( ' release-checkout-flow ' , { key : user .…