# LaunchDarkly SDK: Feature Flag Implementation

# LaunchDarkly SDK: Feature Flag Implementation Guide

## Overview
LaunchDarkly is a feature management platform that enables teams to control feature rollouts, A/B testing, and gradual deployments without code changes. The SDK provides real-time feature flag evaluation.

## Core Concepts

### Feature Flags
- **Boolean Flags**: Simple on/off toggles
- **Multivariate Flags**: Multiple variations (strings, numbers, JSON objects)
- **Targeting Rules**: Segment users based on attributes
- **Rollout Percentages**: Gradual user exposure

### Key Components
1. **SDK Client**: Initializes connection to LaunchDarkly
2. **User Context**: Identifies and attributes users
3. **Flag Evaluation**: Retrieves flag values for users
4. **Event Tracking**: Sends analytics to LaunchDarkly

## Implementation Steps

### 1. Installation
```bash
npm install launchdarkly-js-client-sdk
# or
yarn add launchdarkly-js-client-sdk
```

### 2. Initialize SDK
```javascript
import * as LDClient from 'launchdarkly-js-client-sdk';

const client = LDClient.initialize('YOUR_CLIENT_SIDE_ID', {
  user: {
    key: 'user-123',
    name: 'John Doe',
    email: 'john@example.com',
    custom: {
      plan: 'premium',
      region: 'us-west'
    }
  }
});

// Wait for SDK to be ready
await client.waitForInitialization();
```

### 3. Evaluate Flags
```javascript
// Boolean flag
const showNewFeature = client.variation('new-feature-flag', false);

// Multivariate flag
const uiTheme = client.variation('ui-theme', 'light');

// With details
const flagDetail = client.variationDetail('feature-flag', false);
console.log(flagDetail.value);
console.log(flagDetail.reason); // Why this value was returned
```

### 4. Listen for Changes
```javascript
client.on('change:feature-flag', (newValue, oldValue) => {
  console.log(`Flag changed from ${oldValue} to ${newValue}`);
  // Update UI reactively
});

client.on('change', () => {
  console.log('Any flag changed');
});
```

### 5. Update User Context
```javascript
await client.identify({
  key: 'user-456',
  name: 'Jane Smith',
  custom: {
    plan: 'enterprise'
  }
});
```

## React Integration Example

```javascript
import { useFlags, useLDClient } from 'launchdarkly-react-client-sdk';

function App() {
  const { newFeature, betaUI } = useFlags();
  const ldClient = useLDClient();

  return (
    <div>
      {newFeature && <NewFeatureComponent />}
      {betaUI ? <BetaUI /> : <StandardUI />}
      
      <button onClick={() => ldClient.track('user-action')}>
        Track Event
      </button>
    </div>
  );
}
```

## Advanced Features

### Targeting Rules
```javascript
// In LaunchDarkly dashboard, create rules like:
// - Target specific users
// - Target by custom attributes (plan, region, etc.)
// - Percentage rollouts
// - Scheduled rollouts
```

### A/B Testing
```javascript
const variant = client.variation('ab-test-flag', 'control');

if (variant === 'treatment') {
  // Show treatment version
} else {
  // Show control version
}

// Track conversion
client.track('purchase', { value: 99.99 });
```

### Custom Events
```javascript
client.track('feature-used', {
  featureName: 'new-dashboard',
  timestamp: Date.now()
});

client.track('conversion', { revenue: 150 }, 150);
```

## Best Practices

1. **Initialize Early**: Set up SDK before rendering UI
2. **Handle Offline**: Provide sensible defaults
3. **Segment Users**: Use custom attributes for targeting
4. **Track Events**: Monitor feature adoption
5. **Clean Up**: Remove listeners when components unmount
6. **Cache Flags**: Store evaluated flags for performance
7. **Error Handling**: Implement fallbacks for SDK failures

## Common Patterns

### Feature Rollout
```javascript
// Start with 0%, gradually increase to 100%
// LaunchDarkly handles percentage-based targeting
const isEnabled = client.variation('gradual-rollout', false);
```

### Kill Switch
```javascript
const isServiceDown = client.variation('kill-switch', false);
if (isServiceDown) {
  return <MaintenancePage />;
}
```

### Experimentation
```javascript
const experiment = client.variation('experiment-flag', 'control');
trackExperimentEvent(experiment);
```

## Troubleshooting

| Issue | Solution |
|-------|----------|
| Flags not updating | Ensure `waitForInitialization()` completes |
| Wrong flag values | Verify user context attributes match targeting rules |
| Events not tracked | Check network connectivity and event tracking enabled |
| Performance issues | Implement flag caching and lazy evaluation |

## Resources
- [LaunchDarkly Documentation](https://docs.launchdarkly.com)
- [SDK Reference](https://launchdarkly.com/sdk)
- [Best Practices Guide](https://docs.launchdarkly.com/guides/best-practices)

This implementation enables powerful feature management with minimal code changes and maximum control over deployments.