Node.js SDK for Featureflip - evaluate feature flags locally with near-zero latency. Built on top of @featureflip/js with Node.js-specific defaults.

Using OpenFeature? See @featureflip/openfeature-node, the Featureflip provider for the OpenFeature Node.js SDK.

Installation

npm install @featureflip/node

Quick Start

import { FeatureflipClient } from '@featureflip/node';

// Blocks until flags are loaded
const client = await FeatureflipClient.create({
  sdkKey: 'your-sdk-key',
});

const enabled = client.boolVariation('my-feature', { user_id: '123' }, false);

if (enabled) {
  console.log('Feature is enabled!');
}

await client.close();

Or, obtain a handle synchronously and wait manually:

const client = FeatureflipClient.get({ sdkKey: 'your-sdk-key' });
await client.waitForInitialization();

Singleton by construction. FeatureflipClient.get() is the only way to obtain a client — the public constructor was removed in v2.0. Calling get() more than once with the same SDK key returns handles pointing at one shared underlying client. The factory is refcounted, so closing a handle only shuts down the shared core when the last handle is closed. This makes the SDK safe to call from per-request handlers and DI containers without leaking SSE connections.

Configuration

const client = await FeatureflipClient.create({
  sdkKey: 'your-sdk-key',
  baseUrl: 'https://eval.featureflip.io', // Evaluation API URL (default)
  streaming: true,           // Use SSE for real-time updates (default)
  pollInterval: 30000,       // Polling interval in ms if streaming=false
  flushInterval: 30000,      // Event flush interval in ms
  flushBatchSize: 100,       // Events per batch
  initTimeout: 10000,        // Max ms to wait for initialization
  maxStreamRetries: 5,       // SSE retries before falling back to polling
});

Evaluation

const context = { user_id: '123', email: 'user@example.com' };

// Boolean flag
const enabled = client.boolVariation('feature-key', context, false);

// String flag
const tier = client.stringVariation('pricing-tier', context, 'free');

// Number flag
const limit = client.numberVariation('rate-limit', context, 100);

// JSON flag
const config = client.jsonVariation('ui-config', context, { theme: 'light' });

Detailed Evaluation

const detail = client.variationDetail('feature-key', { user_id: '123' }, false);

console.log(detail.value);   // The evaluated value
console.log(detail.reason);  // "RuleMatch", "Fallthrough", "FlagDisabled", etc.
console.log(detail.ruleId);  // Rule ID if reason is "RuleMatch"

Event Tracking

// Track custom events
client.track('checkout-completed', { user_id: '123' }, { total: 99.99 });

// Record an identify event for analytics (does not affect flag evaluation)
client.identify({ user_id: '123', email: 'user@example.com', plan: 'pro' });

// Force flush pending events
await client.flush();

Featureflip counts each event name, per environment. The metadata you pass is transmitted with the event but is not stored, and event counts are not surfaced in the app or API.

Testing

Use forTesting() to create a client with predetermined flag values -- no network calls.

const client = FeatureflipClient.forTesting({
  'my-feature': true,
  'pricing-tier': 'pro',
});

client.boolVariation('my-feature', {}, false);     // true
client.stringVariation('pricing-tier', {}, 'free'); // 'pro'