Feature Flags

LaunchDarkly

import * as LaunchDarkly from '@launchdarkly/node-server-sdk';

const client = LaunchDarkly.init(process.env.LD_SDK_KEY!);
await client.waitForInitialization();

// Evaluate flag
const showNewUI = await client.variation('new-dashboard-ui', {
  key: user.id,
  email: user.email,
  custom: { plan: user.plan },
}, false); // default value

if (showNewUI) {
  renderNewDashboard();
} else {
  renderLegacyDashboard();
}

React SDK

import { useFlags, withLDProvider } from 'launchdarkly-react-client-sdk';

function Dashboard() {
  const { newDashboardUi, maxUploadSize } = useFlags();

  return newDashboardUi ? <NewDashboard maxUpload={maxUploadSize} /> : <LegacyDashboard />;
}

// Wrap app
export default withLDProvider({
  clientSideID: process.env.NEXT_PUBLIC_LD_CLIENT_ID!,
  context: { kind: 'user', key: user.id, email: user.email },
})(App);

Unleash (self-hosted)

import { initialize } from 'unleash-client';

const unleash = initialize({
  url: 'https://unleash.example.com/api',
  appName: 'my-app',
  customHeaders: { Authorization: process.env.UNLEASH_API_KEY! },
});

if (unleash.isEnabled('new-checkout', { userId: user.id })) {
  // New checkout flow
}

Custom Implementation (simple)

interface FeatureFlags {
  [key: string]: {
    enabled: boolean;
    rolloutPercentage?: number;
    allowedUsers?: string[];
  };
}

class FlagService {
  constructor(private flags: FeatureFlags) {}

  isEnabled(flag: string, userId?: string): boolean {
    const config = this.flags[flag];
    if (!config?.enabled) return false;
    if (config.allowedUsers?.includes(userId!)) return true;
    if (config.rolloutPercentage != null && userId) {
      const hash = simpleHash(userId + flag) % 100;
      return hash < config.rolloutPercentage;
    }
    return config.enabled;
  }
}

Flag Types

Anti-Patterns

Production Checklist

• Default values for all flags (graceful degradation)
• Flag naming convention (feature-name-action)
• Cleanup process for fully-rolled-out flags
• Monitoring: flag evaluation metrics
• Testing both flag states in CI
• Audit log of flag changes