How do you implement feature flags in a Node.js application?

Question

Accepted Answer

**Feature flags** — enable/disable features without deploying.

```javascript
// Simple feature flag service
class FeatureFlags {
    constructor() {
        this.flags = new Map();
    }

async init() {
        const flags = await fetchFlags();
        flags.forEach(f => this.flags.set(f.key, f));
    }

isEnabled(flagKey, context = {}) {
        const flag = this.flags.get(flagKey);
        if (!flag) return false;

// Percentage rollout
        if (flag.percentage !== undefined) {
            const hash = hashCode(context.userId || "anonymous");
            return (hash % 100) < flag.percentage;
        }

return flag.enabled;
    }
}

const features = new FeatureFlags();
await features.init();

// Usage in route
app.get("/dashboard", (req, res) => {
    const showNewUI = features.isEnabled("new-dashboard", {
        userId: req.user.id,
    });

if (showNewUI) {
        return res.render("dashboard-v2");
    }
    res.render("dashboard-v1");
});
```

**Services:**
- LaunchDarkly, Flagsmith, Unleash.
- Gradual rollouts, A/B testing.

Node.js: How do you implement feature flags in a Node.js application?