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name: launchdarkly description: >- Manage feature flags with LaunchDarkly. Use when a user asks to implement feature flags, roll out features gradually, do A/B testing, control feature access by user segment, or implement kill switches for features. license: Apache-2.0 compatibility: 'Any language — JS, Python, Go, Java, Ruby, .NET SDKs' metadata: author: terminal-skills version: 1.0.0 category: development tags: - launchdarkly - feature-flags - feature-toggles - ab-testing - rollout

LaunchDarkly

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

npm install @launchdarkly/node-server-sdk     # server
npm install launchdarkly-react-client-sdk      # React

Step 2: Server-Side Flags

// 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

// 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.