Edge Runtime

🔍 What Is It?

The Edge Runtime is a lightweight JavaScript runtime that:

• →Runs at CDN edge nodes — 100+ locations globally, close to users
• →Uses Web APIs — fetch, Request, Response, Headers, crypto (Web Crypto)
• →Does NOT support Node.js APIs — no fs, no net, no most npm packages
• →Starts instantly — no cold starts (unlike Node.js serverless functions)
• →Has memory limits — 128MB max (vs 1GB+ for Node.js functions)

Opt into Edge Runtime:

export const runtime = 'edge'; // In a Route Handler or page

🎯 Why Do We Need It?

Node.js serverless functions have two problems:

1. Cold starts — first request after idle period takes 200–800ms to initialise
2. Single region — a function in us-east-1 serves users in Tokyo with 150ms latency

Edge Runtime solves both:

• →No cold starts — always warm
• →Globally distributed — Tokyo users served from Tokyo edge node

🏗️ What Happens in Real Projects?

Edge Runtime use cases:

• →Middleware — auth redirects, A/B testing (always Edge)
• →Personalisation — serve different content based on geolocation
• →Simple API endpoints — no DB access, pure computation
• →Rate limiting — count requests at the edge with Upstash Redis
• →JWT verification — verify tokens without a DB lookup

Do NOT use Edge Runtime for:

• →Database queries (most drivers require Node.js)
• →File system operations
• →Heavy computation (memory limits)

⚙️ How It Works

Edge vs Node.js runtime comparison:

💻 Example

Edge Runtime Patterns — Edge Route Handler and geolocation-based personalisation.

// app/api/hello/route.ts
export const runtime = 'edge'; // Opt into Edge Runtime

export async function GET(request: Request) {
  // Geolocation from Vercel edge headers
  const country = request.headers.get('x-vercel-ip-country') ?? 'US';
  const city = request.headers.get('x-vercel-ip-city') ?? 'Unknown';

  // Web Crypto API (not Node.js crypto)
  const id = crypto.randomUUID();

  return Response.json({
    message: `Hello from ${city}, ${country}!`,
    id,
    runtime: 'edge',
  });
}

🤔 Common Confusions

🚨 Common Production Issues

✅ Best Practices

🏛️ Architect's Note

💬 Frequently Asked Questions

🎯 Key Takeaways

🔗 Related Topics

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↗ Where this topic fits