Edge Functions for Website (Vercel Edge)

Edge Functions Development for Websites (Vercel Edge)

Vercel Edge Functions run on the Vercel Edge Network — on 100+ nodes worldwide, milliseconds from the user. Cold start less than 1 ms. This distinguishes them from regular Vercel Functions, which run in a single AWS region.

When Edge, When Regular Functions

Edge Functions are optimal for: personalization (A/B testing, geolocation), redirects and rewrites based on conditions, middleware authentication, header and response transformation, cache validation by cookie.

Regular Node.js Functions are needed when: Node.js API is required (fs, crypto, native modules), TCP connection to PostgreSQL, execution time > 30 s, memory > 128 MB.

Edge Runtime uses Web API (like in browsers), not Node.js API.

Middleware for A/B Testing

File middleware.ts in the root of Next.js project:

import { NextRequest, NextResponse } from "next/server";

export function middleware(request: NextRequest) {
  const url = request.nextUrl.clone();

  // A/B test for homepage
  if (url.pathname === "/") {
    const bucket = request.cookies.get("ab-bucket")?.value;

    if (!bucket) {
      const newBucket = Math.random() < 0.5 ? "a" : "b";
      const response = NextResponse.rewrite(
        new URL(newBucket === "b" ? "/home-variant" : "/", request.url)
      );
      response.cookies.set("ab-bucket", newBucket, { maxAge: 86400 * 30 });
      return response;
    }

    if (bucket === "b") {
      url.pathname = "/home-variant";
      return NextResponse.rewrite(url);
    }
  }

  return NextResponse.next();
}

export const config = {
  matcher: ["/", "/pricing", "/features"],
};

Geolocation and Personalization

import { NextRequest, NextResponse } from "next/server";
import { geolocation } from "@vercel/functions";

export function middleware(request: NextRequest) {
  const { country, city } = geolocation(request);

  // Redirect to localized version
  if (country === "RU" && !request.nextUrl.pathname.startsWith("/ru")) {
    return NextResponse.redirect(
      new URL(`/ru${request.nextUrl.pathname}`, request.url)
    );
  }

  // Add geo data to headers for components
  const response = NextResponse.next();
  response.headers.set("x-user-country", country || "unknown");
  response.headers.set("x-user-city", city || "unknown");
  return response;
}

Edge API Route

// app/api/edge-data/route.ts
import { NextRequest, NextResponse } from "next/server";

export const runtime = "edge";

export async function GET(request: NextRequest) {
  const { searchParams } = new URL(request.url);
  const id = searchParams.get("id");

  // Fetch works natively in Edge Runtime
  const data = await fetch(`https://api.external.com/data/${id}`, {
    headers: { Authorization: `Bearer ${process.env.API_KEY}` },
    // next.js cache: cache for 60 seconds
    next: { revalidate: 60 },
  }).then(r => r.json());

  return NextResponse.json(data, {
    headers: { "Cache-Control": "s-maxage=60, stale-while-revalidate=120" }
  });
}

Route Protection via JWT

import { NextRequest, NextResponse } from "next/server";
import { jwtVerify } from "jose";

const JWT_SECRET = new TextEncoder().encode(process.env.JWT_SECRET);

export async function middleware(request: NextRequest) {
  if (request.nextUrl.pathname.startsWith("/dashboard")) {
    const token = request.cookies.get("auth-token")?.value;

    if (!token) {
      return NextResponse.redirect(new URL("/login", request.url));
    }

    try {
      await jwtVerify(token, JWT_SECRET);
      return NextResponse.next();
    } catch {
      const response = NextResponse.redirect(new URL("/login", request.url));
      response.cookies.delete("auth-token");
      return response;
    }
  }
}

jose is the only JWT library compatible with Edge Runtime (uses Web Crypto API instead of node:crypto).

Edge Runtime Limitations

• No node:fs, node:path, node:crypto (Web Crypto available)
• No native npm packages
• Memory: 128 MB
• CPU time: 30 ms (Hobby), unlimited on Pro
• No direct connections to PostgreSQL (Neon and PlanetScale support HTTP API)

Timeframe

Middleware with geolocation and A/B testing — 1–2 days. Edge API Routes with caching and JWT protection — 2–3 days.