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

Feature	Edge Runtime	Node.js (Serverless)
Cold start	None (~0ms)	250ms–1s+
Max duration	30s (Hobby), 5min (Pro)	10s (Hobby), 5min (Pro)
Max size	1 MB (after gzip)	250 MB (unzipped)
APIs available	Web Standard APIs	Full Node.js API
npm packages	Limited (no native modules)	All npm packages
Global deployment	Automatic	Single region default
Use case	Auth, routing, A/B, geo	Database queries, heavy compute

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

Error	Cause	Solution
`EDGE_FUNCTION_INVOCATION_FAILED`	Runtime error in edge function	Check logs — no `try/catch` around async code
`Dynamic Code Evaluation not allowed`	Using `eval()` or `new Function()`	Refactor to avoid dynamic code evaluation
`Module not found`	npm package uses Node.js APIs	Use edge-compatible alternative or switch to Node.js runtime
`FUNCTION_PAYLOAD_TOO_LARGE`	Edge function bundle > 1 MB	Tree-shake imports, split into smaller functions
`TypeError: x is not a function`	Node.js API used in edge runtime	Replace with Web Standard API equivalent

Resources

Next Steps

For common errors, see vercel-common-errors.