LaunchDarkly JavaScript SDK Simplifies Feature Flags

Introduction

Feature flags allow developers to toggle parts of an application on or off without re-deploying code. LaunchDarkly provides a feature flag management platform that makes it easy to control these flags across environments. Their JavaScript SDK integrates directly with client-side JavaScript code to evaluate flags and manage app features remotely.

The LaunchDarkly JS SDK offers several benefits for implementing feature flags in JavaScript apps:

Quick and simple setup - just install the SDK and initialize with your project ID
Target users or groups with custom rules to selectively enable features
Track usage analytics and custom events for user behavior insights
Flexible flag variations beyond booleans like strings or numbers
Local feature flag fallback and caching to prevent errors
Built-in support for popular frameworks like React, Angular, and Vue

With the SDK, developers can easily tap into LaunchDarkly's platform to control feature releases, conduct effective A/B tests, manage dark launches, and more. The SDK acts as a client-side gateway to LaunchDarkly, greatly simplifying access to feature flags from JavaScript code.

Getting Started with the LaunchDarkly JS SDK

Getting started with the LaunchDarkly JavaScript SDK only takes a few simple steps. First, you'll need to install the SDK into your project. Then initialize a new client with your LaunchDarkly project ID and user.

Installation

For a React project using NPM and Webpack:

npm install launchdarkly-js-client-sdk

Then import the SDK:

import * as LDClient from 'launchdarkly-js-client-sdk';

This integrates the LaunchDarkly SDK for use in your React components.

For a simple HTML page, reference the SDK from a CDN:

<script src="https://cdn.jsdelivr.net/npm/launchdarkly-js-client-sdk@3.0.0"></script>

NPM is best for bundlers while CDN works for basic scripts.

Initialization

To initialize the SDK client:

const ldClient = LDClient.initialize('YOUR_PROJECT_ID', {
  key: 'user123' 
});

For anonymous users, generate a unique user key:

const ldClient = LDClient.initialize('YOUR_PROJECT_ID', {
  key: uuidv4()
});

Initialize early in your app to load flags before use.

Build Formats

The SDK supports ES modules, CommonJS, and UMD formats. Use ES modules with Webpack for tree shaking. Go with CommonJS for Node.js and UMD builds directly in the browser.

Using Feature Flags

Once initialized, you can evaluate flags and variations. Target users, create custom rules, and return any data type - not just booleans.

Flag Evaluation

Check if a feature flag is enabled:

const showFeature = ldClient.variation('show-feature', false);

if (showFeature) {
  // Flag is enabled
} else {
  // Flag is disabled  
}

The second parameter provides a fallback value if the flag fails.

User Targeting

Target flags to specific users:

ldClient.variation('show-feature', false, {
  key: 'user123',
  custom: {
    premium: true
  }
})

You can target by user properties, groups, or custom rules.

Flag Variations

Flag variations can return any data type - not just booleans:

const plan = ldClient.variation('pricing-plan', 'free');

if (plan === 'premium') {
  // Show premium version
} else {
  // Show free version
}

Even complex JSON objects:

const price = ldClient.variation('price', {amount: 0});

showPrice(price.amount);

This allows greater control over feature rollouts.

Analytics and Events

The LaunchDarkly SDK captures usage analytics automatically. You can also track custom events like page views, clicks, form submissions, and more.

Usage Data

By default, the SDK tracks feature flag usage and adoption. Disable tracking if desired:

const ldClient = LDClient.initialize('YOUR_PROJECT_ID', {
  sendEvents: false
});

View usage analytics in your LaunchDarkly project dashboard.

Custom Events

Log custom events to track user behaviors:

ldClient.track('pageview', {
  path: '/pricing' 
});

Capture clicks, purchases, signups, or anything else:

const button = document.getElementById('signup');

button.addEventListener('click', () => {
  ldClient.track('signup', {
    plan: 'Pro Plan'
  });
});

Performance Monitoring

Use timers to monitor backend calls and third-party services:

const timer = ldClient.startTimer('api-call');

// API request...

ldClient.stopTimer(timer);

View performance metrics in your LaunchDarkly dashboard.

Best Practices

Here are some tips for integrating LaunchDarkly:

Initialize early in app startup
Implement failover defaults for flags
Fetch initial state on server to prevent flicker
Enable private user attributes for consistent behavior
Set up analytics reporting integrations
Only request flags as needed with allFlagsState
Use DevHunt to showcase new features

Conclusion

The LaunchDarkly JavaScript SDK makes it easy to use feature flags in your JavaScript app. With the ability to target users, create custom rules, track analytics, and more, you can manage feature releases with precision.

If you're looking to implement advanced feature management, be sure to check out the LaunchDarkly SDK on DevHunt to see how it can help you build better software!

Categories