Vercel Edge Functions - Run Code at the Edge

Edge Functions are JavaScript code that runs at edge locations worldwide instead of in one big data center. Think of it like having your backend code scattered around the globe, closer to your users.

They boot faster than regular serverless because there's no Docker overhead slowing things down. Built on V8 (same engine as Chrome) which means they start instantly instead of waiting for containers to spin up.

What You Actually Get (Spoiler: Less Than You Think)

You get fetch, Request, Response, Headers. That's it. No filesystem access, no Node.js modules, no eval(). Functions must start responding within 25 seconds but can keep streaming for up to 300 seconds.

Here's the catch: you can't access the filesystem or run dynamic code. This means half your favorite Node.js packages won't work. The 1MB limit includes dependencies, so importing lodash will fuck you over. The 1MB limit will bite you when you import one medium-sized library and suddenly your function is 1.2MB.

Where Your Code Actually Runs

Edge Functions run in the region closest to your user, which usually means faster response times. You can force functions to run in specific regions using preferredRegion if you need to be close to your database.

Pro tip: If your database is in us-east-1 but your users are global, you'll still get faster auth checks by running at the edge and making one quick database call instead of bouncing everything through Virginia.

How It Works with Next.js

Edge Functions work with Next.js out of the box. You can use them as middleware (to modify requests/responses) or as API routes. Just add export const runtime = 'edge' to your function and Vercel handles the rest.

Deployment is automatic through Git integration. Push to main, wait 30 seconds, your edge function is live globally. No Docker files, no deployment configs, no bullshit.

You only pay for CPU time when your function is actually doing work, not just sitting there.

When Edge Functions Don't Suck

They're perfect for:

• A/B testing and feature flags (change user experience instantly)
• Authentication middleware (protect routes without hitting your backend)
• API rate limiting (throttle requests before they reach your servers)
• Simple data transformations (format responses on the fly)

Don't use them for complex database operations or anything that needs Node.js libraries. Regular Vercel Functions are better for that stuff.

What Will Break Your Day

Edge Functions are great until they're not. Here's what'll catch you:

• JWT validation works great until the edge runtime reminds you that crypto.subtle doesn't exist and your entire auth flow explodes - Had to switch from jsonwebtoken to jose because crypto APIs are different
• The streaming support is cool but debugging streaming failures will make you question your career choices - Error messages are useless when streams fail halfway through
• We tried using Edge Functions for image processing. The 128MB memory limit laughed at our JPEG processing. Learned that lesson the expensive way.

Authentication and Authorization Middleware

Edge Functions are solid for authentication workflows because they run closer to users. JWT validation, session checks, and OAuth redirects happen faster when they don't need to round-trip to your main backend.

export const runtime = 'edge';

export async function middleware(request: Request) {
  const token = request.headers.get('authorization');

  if (!token || !await validateToken(token)) {
    return new Response('Unauthorized', { status: 401 });
  }

  return NextResponse.next();
}

We've seen this pattern work in production to protect API endpoints and admin interfaces without the latency hit.

But here's what breaks: JWT libraries that work in Node.js often fail in the edge runtime. Had to switch from jsonwebtoken to jose because crypto APIs are different.

A/B Testing and Feature Flags

Edge Functions are great for A/B testing because you can change user experiences instantly without redeploying your app. Check user segments, location, device type, whatever - and serve different content on the fly.

A/B tests run faster at the edge because there's less latency when switching between variants. Edge Config lets you update feature flags instantly across all edge locations without touching your code.

Gotcha: Edge Config has sync delays. Sometimes your feature flag changes take 30-60 seconds to propagate globally. Plan for this or you'll get confused why your changes aren't showing up immediately.

API Rate Limiting and Request Transformation

Edge Functions make global rate limiting easier because you can block requests before they hit your backend. Rate limiting sounds simple but storing state between requests is a nightmare - Edge Functions don't persist data between invocations.

// Rate limiting implementation
const rateLimiter = new Map();

export function GET(request: Request) {
  const clientIP = request.headers.get('x-forwarded-for');
  const currentRequests = rateLimiter.get(clientIP) || 0;

  if (currentRequests > 100) {
    return new Response('Rate limit exceeded', { status: 429 });
  }

  rateLimiter.set(clientIP, currentRequests + 1);
  return processRequest(request);
}

Real-Time Data Processing and Webhooks

The 300-second execution limit makes Edge Functions useful for webhook processing and real-time analytics. You can handle payment callbacks, user events, and third-party integrations without running dedicated servers.

Financial companies use edge functions for transaction processing and fraud detection because the code runs closer to where the transactions happen.

Reality check: Webhook processing works great until you need to validate signatures. Half the webhook libraries were written by people who never heard of edge computing and expect Node.js crypto APIs that don't exist. You'll waste 3 hours wondering why crypto.createHmac is undefined, then another 2 hours finding the right edge-compatible crypto library that actually works.

Content Personalization and Dynamic Routing

Edge Functions let you personalize content based on geography, device type, or user preferences without hitting your main backend. This reduces CDN cache misses because you're serving more relevant content.

Page loads get faster when personalization happens at the edge instead of bouncing back to your origin server for every request.

Warning that'll save your ass: Cold starts are a lie. Sometimes your function decides to take a 2-second coffee break because the V8 engine felt like it. Your monitoring will go apeshit and you'll never know why. Plan for random latency spikes or enjoy getting paged at 3am.