JavaScript feature flags: Client-side patterns

You know that sinking feeling when you push a broken feature to production and can't roll it back without a full deployment? Client-side feature flags solve that problem - they let you control what users see without touching your code.

Think of them as switches you can flip remotely. Need to hide that buggy checkout flow? Done. Want to test a new design with 10% of users? Easy. The best part is that JavaScript makes this incredibly straightforward, whether you're working with a static site or a complex React app.

Understanding client-side feature flags in JavaScript applications

are basically conditional statements on steroids. Instead of hardcoding if (showNewFeature), you're pulling that decision from a service that you control. This means you can ship code that's ready but hidden, test features with specific users, and kill problematic features instantly without redeploying.

Client-side flags work by downloading configuration to the user's browser and caching it. Your JavaScript checks these cached values to decide what to show. It's fast because there's no network round trip for each check - though you do need a way to refresh those values when things change.

Here's where they really shine:

• A/B testing: Ship two versions of a feature and see which one actually moves the needle

• Gradual rollouts: Start with 1% of users, watch your metrics, then slowly ramp up

• User personalization: Show premium features to paid users without separate codebases

The handles the heavy lifting here. It manages the connection to your flag service, caches values locally, and gives you clean APIs to check flag states. Most importantly, it provides typed getters so you're not casting strings to booleans all over your codebase.

But let's be honest - . You end up with conditional logic scattered throughout your components. Dead code piles up. And if your , you'll have flags in one place that don't match the other. The key is treating flags as temporary by default and having a plan to remove them.

Implementing feature flags on static sites and single-page applications

Static sites

Static sites seem like they'd be terrible for feature flags - after all, they're static, right? Not really. You just need to handle everything client-side.

The trick is loading your JavaScript SDK early. Like, really early. Put it in the head tag before any other scripts. This prevents that awkward flash where users see the old version before flags kick in. Bootstrapping helps too - you can pass initial flag values directly to avoid that first network request.

For real-time updates, subscribe to flag changes. When someone flips a switch in your flag dashboard, connected clients get the update and can react immediately. No page refresh needed.

Single-page applications

SPAs and feature flags are a natural fit. Your app's already managing state client-side, so adding flag state is straightforward. React developers often wrap their flag logic in context providers or custom hooks to keep components clean.

The challenge is timing. You want flags loaded before your components render, or you'll get flashing and layout shifts. Most experienced developers solve this by initializing flags in their app's entry point and delaying the initial render until flags are ready.

Coordination between teams matters here. As one developer put it, having front-end flags that don't match back-end behavior creates a terrible user experience. Some teams are even building tools that let developers toggle flags right in their dev environment.

When you combine flags with modern SPA patterns like code splitting, you can load features on demand based on flag state. User doesn't have access to the admin panel? Don't even download that code.

Optimizing performance with bootstrapping and delayed rendering

Bootstrapping feature flags

Nobody likes waiting. That's why bootstrapping is crucial - it eliminates the initial flag fetch that blocks your app startup.

Here's how it works: your server includes the user's flag values in the initial HTML response. When your JavaScript SDK initializes, it uses these values immediately instead of making a network request. This cuts your startup time from seconds to milliseconds.

The implementation is simple. Generate the bootstrap values server-side, inject them as a script tag, then pass them to your SDK during initialization. Tools like Statsig handle this pattern elegantly, letting you generate bootstrap values that work seamlessly with their JavaScript SDK.

Delayed rendering and real-time updates

Sometimes it's better to wait a moment than show the wrong thing. Delayed rendering prevents that jarring experience where users see one version of your site, then it suddenly changes.

The pattern looks like this:

1. Initialize your SDK and flags

2. Show a minimal loading state

3. Render your full UI once flags are ready

4. Subscribe to updates for live changes

React developers often handle this with a simple flag provider that blocks child rendering until initialization completes. The JavaScript SDK typically provides callbacks or promises to signal when it's ready.

Real-time updates keep your app responsive to flag changes. When someone adjusts a flag value, all connected clients update automatically. This is perfect for coordinating feature launches or quickly disabling problematic features. The LaunchDarkly approach uses server-sent events or polling to push updates to clients.

Performance doesn't stop at initialization. Combine flags with code splitting to load only what users need. Use prefetching to prepare features before they're enabled. Every millisecond counts when you're competing for user attention.

Advanced patterns and best practices in client-side feature flagging

Integrating with React applications

React and feature flags work beautifully together, but you need the right setup. The React Web SDK pattern that works best wraps your entire app in a provider that manages flag state.

Here's the approach that actually works in production:

• Create a context that holds your flag client

• Initialize flags before rendering your app

• Use hooks to access flags in components

• Keep flag checks close to where they're used

The JavaScript SDK integration becomes even cleaner with TypeScript. You get autocomplete for flag names and type safety for flag values. No more runtime errors from typos in flag names.

One pattern I've seen work well is creating feature-specific components that handle their own flag logic internally. Instead of {flag && <Feature />} scattered everywhere, you have <Feature /> that knows how to check its own visibility. Statsig's SDK makes this particularly clean with their typed configuration values.

Security and testing considerations

Never put secrets in client-side flags. Seriously. Everything in your JavaScript bundle is public. If you need to gate sensitive features, do the check server-side and only send down what the user should see.

Experienced developers keep a few rules in mind:

• Client flags control UI, server flags control data

• Document every flag's purpose and removal date

• Use descriptive names that explain what "on" means

• Clean up old flags religiously

Testing gets tricky with flags. You can't test every possible combination - with 10 flags, that's over 1,000 scenarios. Instead, focus on:

1. The current production state

2. Each new feature in isolation

3. The planned next release state

Smart teams use parallel data fetching to speed up testing. Load your test data while flags initialize, not after. Cache aggressively in development to avoid waiting for flag updates during rapid iteration.

The best practice? Treat flags as temporary. They're for releases and experiments, not permanent configuration. Once a feature is stable and rolled out to everyone, remove the flag and its associated code. Your future self will thank you.

Closing thoughts

Client-side feature flags give you superpowers - the ability to control your app's behavior without deployments, test features safely, and personalize experiences at scale. But like any powerful tool, they need to be used thoughtfully.

Start simple. Pick one feature to flag. Get comfortable with the SDK and the workflow. Then gradually expand as you see the benefits. Whether you're using LaunchDarkly, Statsig, or rolling your own solution, the patterns stay the same: initialize early, cache aggressively, and always have a plan to remove flags once they've served their purpose.

Want to dive deeper? Check out Martin Fowler's feature flag patterns, explore how teams at scale handle flags, or just start experimenting with a simple flag in your next project. The best way to learn is by doing.

Hope you find this useful!