Node.js Feature Flags

3. Evaluate flags

const fetch = require('node-fetch');

const FLAGBIT_SDK_KEY = process.env.FLAGBIT_SDK_KEY;

async function getFlag(flagKey, context = {}) {
  try {
    const res = await fetch(
      'https://flagbit.anethoth.com/api/v1/evaluate', {
      method: 'POST',
      headers: {
        'Content-Type': 'application/json',
        'X-SDK-Key': FLAGBIT_SDK_KEY
      },
      body: JSON.stringify({ flag_key: flagKey, context })
    });
    const data = await res.json();
    return data.value;
  } catch {
    return false; // Safe default
  }
}

// Express middleware
function featureGate(flagKey) {
  return async (req, res, next) => {
    const enabled = await getFlag(flagKey, {
      user_id: req.user?.id,
      plan: req.user?.plan
    });
    req.flags = { ...req.flags, [flagKey]: enabled };
    next();
  };
}

// Usage in Express routes
const express = require('express');
const app = express();

app.get('/api/search',
  featureGate('elastic-search'),
  async (req, res) => {
    if (req.flags['elastic-search']) {
      return res.json(await elasticSearch(req.query.q));
    }
    return res.json(await sqlSearch(req.query.q));
  }
);

// NestJS guard
@Injectable()
class FeatureFlagGuard implements CanActivate {
  async canActivate(context) {
    const flagKey = Reflect.getMetadata('feature_flag', context.getHandler());
    return await getFlag(flagKey);
  }
}