Feature flags like `__DEV__` and `__COMPAT__` are critical configuration mechanisms that control environment-specific behavior. Always wrap development-only code (such as warnings and debug information) in `__DEV__` conditionals to ensure they don't ship to production builds. Remember that these flags are replaced at build time through tree-shaking, so their proper usage directly impacts bundle size and performance. ```js // Good: Warning only shown in development if (__DEV__ && unexpectedCondition) { console.warn('This warning helps developers but is removed in production') } // Good: Feature flag affects code path selection if (!(__COMPAT__ && compatFeature)) { // Modern implementation } else { // Compatibility implementation } // Bad: Warning always included in production builds if (unexpectedCondition) { console.warn('This warning will bloat production builds') } ``` This practice ensures configuration is properly managed across different build environments, keeping production code lean while providing helpful development features.