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