Overview
LaunchDarkly is the industry-standard feature flag platform. Control feature rollouts, A/B tests, and entitlements without deploying new code. Supports gradual rollouts, targeting rules, and instant kill switches.
Instructions
Step 1: Setup
bash
npm install @launchdarkly/node-server-sdk # server
npm install launchdarkly-react-client-sdk # React
Step 2: Server-Side Flags
typescript
// lib/flags.ts — Feature flag evaluation on the server
import * as LaunchDarkly from '@launchdarkly/node-server-sdk'
const client = LaunchDarkly.init(process.env.LAUNCHDARKLY_SDK_KEY!)
await client.waitForInitialization()
export async function getFlag(flagKey: string, user: { key: string; email?: string; plan?: string }) {
const context = {
kind: 'user',
key: user.key,
email: user.email,
custom: { plan: user.plan },
}
return client.variation(flagKey, context, false) // false = default
}
// Usage
const showNewDashboard = await getFlag('new-dashboard', { key: userId, plan: 'pro' })
if (showNewDashboard) {
return renderNewDashboard()
}
Step 3: React Client
tsx
// components/FeatureFlag.tsx — Client-side feature flags
import { withLDProvider, useFlags } from 'launchdarkly-react-client-sdk'
function App() {
const { newCheckout, darkMode } = useFlags()
return (
<div className={darkMode ? 'dark' : ''}>
{newCheckout ? <NewCheckout /> : <OldCheckout />}
</div>
)
}
export default withLDProvider({
clientSideID: process.env.NEXT_PUBLIC_LD_CLIENT_ID!,
context: { kind: 'user', key: userId, email: userEmail },
})(App)
Step 4: Gradual Rollout
Configure in LaunchDarkly dashboard:
- Percentage rollout: 10% → 25% → 50% → 100% over days/weeks
- Targeting rules: enable for beta users first, then pro plan, then everyone
- Kill switch: instantly disable a feature if it causes issues
Guidelines
- LaunchDarkly evaluates flags locally (SDK caches rules) — near-zero latency.
- Use multi-variate flags for A/B testing (not just boolean on/off).
- Pricing starts at $10/seat/month. For free alternative, consider Unleash (open-source).
- Always set meaningful defaults — the app should work even if LaunchDarkly is unreachable.