Server-Side Feature Flags with Node.js - Complete Backend Implementation Guide

Node.js backend applications require robust feature management for APIs, microservices, and server-side logic. Feature flags provide the perfect solution for controlling backend functionality, managing database migrations, and enabling safe deployments in Node.js environments.

Why Node.js Backends Need Feature Flags

Node.js applications power millions of APIs and microservices worldwide. Feature flags in Node.js enable:

• Zero-downtime deployments of new API endpoints
• Database migration safety with gradual rollouts
• API versioning without maintaining multiple codebases
• Kill switches for problematic features
• A/B testing at the API level

const { FlagpoleClient } = require("@flagpole/node");

const client = new FlagpoleClient({
  apiKey: "fp_live_your_api_key",
  environments: ["production"],
  fallbacks: {
    "new-payment-gateway": false,
    "enhanced-analytics": false,
  },
});

await client.initialize();

// Safe feature rollout
if (await client.isFeatureEnabled("new-payment-gateway")) {
  return await processPaymentV2(paymentData);
} else {
  return await processPaymentV1(paymentData);
}

Setting Up Feature Flags in Node.js

Installation

npm install @flagpole/node socket.io-client
# or
yarn add @flagpole/node socket.io-client

Basic Client Setup

// config/flagpole.js
const { FlagpoleClient } = require("@flagpole/node");

const flagpoleClient = new FlagpoleClient({
  apiKey: process.env.FLAGPOLE_API_KEY,
  environments: [process.env.NODE_ENV || "development"],
  cache: {
    enabled: true,
    ttl: 300, // 5 minutes
  },
  fallbacks: {
    "database-migration-v2": false,
    "new-user-endpoint": false,
    "enhanced-logging": true,
  },
  timeout: 5000,
  retries: 3,
});

module.exports = flagpoleClient;

Application Initialization

// app.js
const express = require("express");
const flagpoleClient = require("./config/flagpole");

const app = express();

async function startServer() {
  try {
    // Initialize feature flags before starting server
    await flagpoleClient.initialize();
    console.log("Feature flags initialized successfully");

    // Setup routes and middleware
    setupRoutes(app);

    app.listen(3000, () => {
      console.log("Server running on port 3000");
    });
  } catch (error) {
    console.error("Failed to initialize feature flags:", error);
    // Decide whether to continue with fallbacks or exit
    process.exit(1);
  }
}

startServer();

Express.js Integration

Middleware Setup

const { flagpoleMiddleware,