Vercel Edge Functions

Vercel Edge Functions

Overview

Edge Functions run on Vercel's Edge Network (V8 isolates) close to the user with no cold starts. They use Web Standard APIs (Request, Response, fetch) instead of Node.js APIs. Ideal for authentication, A/B testing, geolocation routing, and low-latency API responses.

Prerequisites

• Completed vercel-install-auth setup
• Familiarity with Web APIs (Request/Response)
• Node.js 18+ for local development

Instructions

Step 1: Create an Edge Function

// api/edge-hello.ts
// Export `runtime = 'edge'` to run on the Edge Runtime
export const config = { runtime: 'edge' };

export default function handler(request: Request): Response {
  return new Response(
    JSON.stringify({
      message: 'Hello from the Edge!',
      region: request.headers.get('x-vercel-ip-city') ?? 'unknown',
      timestamp: Date.now(),
    }),
    {
      status: 200,
      headers: { 'Content-Type': 'application/json' },
    }
  );
}

Step 2: Edge Function with Geolocation

Vercel injects geolocation headers into every edge request:

// api/geo.ts
export const config = { runtime: 'edge' };

export default function handler(request: Request): Response {
  const city = request.headers.get('x-vercel-ip-city') ?? 'unknown';
  const country = request.headers.get('x-vercel-ip-country') ?? 'unknown';
  const region = request.headers.get('x-vercel-ip-country-region') ?? 'unknown';
  const latitude = request.headers.get('x-vercel-ip-latitude');
  const longitude = request.headers.get('x-vercel-ip-longitude');

  return Response.json({
    city: decodeURIComponent(city),
    country,
    region,
    coordinates: latitude && longitude ? { lat: latitude, lng: longitude } : null,
  });
}

Step 3: Edge Middleware (middleware.ts)

Middleware runs before every request and can rewrite, redirect, or add headers:

// middleware.ts (must be at project root or src/)
import { NextRequest, NextResponse } from 'next/server';

export function middleware(request: NextRequest) {
  // Example 1: Redirect based on country
  const country = request.geo?.country ?? 'US';
  if (country === 'DE') {
    return NextResponse.redirect(new URL('/de', request.url));
  }

  // Example 2: A/B testing with cookies
  const bucket = request.cookies.get('ab-bucket')?.value;
  if (!bucket) {
    const response = NextResponse.next();
    response.cookies.set('ab-bucket', Math.random() > 0.5 ? 'a' : 'b');
    return response;
  }

  // Example 3: Add security headers
  const response = NextResponse.next();
  response.headers.set('X-Frame-Options', 'DENY');
  response.headers.set('X-Content-Type-Options', 'nosniff');
  response.headers.set('Referrer-Policy', 'strict-origin-when-cross-origin');
  return response;
}

// Only run on specific paths — skip static assets
export const config = {
  matcher: ['/((?!_next/static|_next/image|favicon.ico).*)'],
};

Step 4: Edge Function with Streaming

// api/stream.ts
export const config = { runtime: 'edge' };

export default function handler(): Response {
  const encoder = new TextEncoder();
  const stream = new ReadableStream({
    async start(controller) {
      for (let i = 0; i < 5; i++) {
        controller.enqueue(encoder.encode(`data: chunk ${i}\n\n`));
        await new Promise(r => setTimeout(r, 500));
      }
      controller.enqueue(encoder.encode('data: [DONE]\n\n'));
      controller.close();
    },
  });

  return new Response(stream, {
    headers: {
      'Content-Type': 'text/event-stream',
      'Cache-Control': 'no-cache',
      Connection: 'keep-alive',
    },
  });
}

Step 5: Edge Config (Key-Value Store)

// api/feature-flags.ts
import { get } from '@vercel/edge-config';

export const config = { runtime: 'edge' };

export default async function handler(): Promise<Response> {
  // Read from Edge Config — sub-millisecond reads at the edge
  const maintenanceMode = await get<boolean>('maintenance_mode');
  const featureFlags = await get<Record<string, boolean>>('feature_flags');

  if (maintenanceMode) {
    return Response.json({ status: 'maintenance' }, { status: 503 });
  }

  return Response.json({ features: featureFlags });
}

Install: npm install @vercel/edge-config

Edge vs Serverless Comparison

Available Web APIs in Edge Runtime

fetch, Request, Response, Headers, URL, URLSearchParams, TextEncoder, TextDecoder, ReadableStream, WritableStream, TransformStream, crypto, atob, btoa, structuredClone, setTimeout, setInterval, AbortController

NOT available: fs, path, child_process, net, http, dns, native Node.js modules

Output

• Edge Function deployed globally with zero cold starts
• Geolocation-based routing using Vercel's injected headers
• Middleware running authentication and A/B tests at the edge
• Streaming responses for real-time data delivery

Error Handling

Examples

Roll out edge authorization middleware safely

Implement an allow-listed middleware matcher that excludes static assets and health checks, and validate its Web API-only dependencies with the Edge runtime in preview. Send synthetic signed and unsigned requests from at least two regions, verify that unauthorized requests receive the intended response without logging credentials, and measure added latency. Release behind a reversible configuration flag; if legitimate traffic is blocked, disable the flag or restore the previous deployment before changing the policy.

Resources

• Edge Functions Documentation
• Edge Middleware
• Edge Config
• Edge Runtime API
• Vercel Geolocation Headers

Next Steps

For common errors, see vercel-common-errors.