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

Type	Purpose	Example
Boolean	On/off toggle	new-ui-enabled
Percentage rollout	Gradual release	10% → 50% → 100%
User targeting	Specific users	Beta testers, internal
Multivariate	A/B/C testing	checkout-variant: "A"
Kill switch	Emergency disable	payments-enabled

Anti-Patterns

Anti-Pattern	Fix
Flags that live forever	Set expiration, clean up after full rollout
Flag checks deep in code	Check at entry points, pass result down
No default values	Always provide sensible defaults
Testing only flagged-on path	Test both paths in CI
Flags in tight loops	Cache flag evaluation result

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