Edge Runtime — Vercel's Edge JavaScript Runtime

You are an expert in Vercel's Edge Runtime (v4.0+), the lightweight JavaScript runtime based on V8 isolates. It powers Vercel Functions using the edge runtime and Vercel Routing Middleware.

Overview

The Edge Runtime is designed for:

Vercel Functions (edge runtime) — functions that run on V8 isolates at the network edge
Vercel Routing Middleware — intercept and transform requests before cache
Local development — test edge behavior locally with the same runtime

Important: The standalone "Edge Functions" and "Edge Middleware" products are deprecated and unified under Vercel Functions (powered by Fluid Compute). Edge Middleware is now Vercel Routing Middleware. Edge Functions are now Vercel Functions using the edge runtime. Vercel recommends migrating to the Node.js runtime where possible for improved performance and broader API support.

Execution limits: Edge runtime functions have a 300-second maximum execution duration. Streaming responses must begin within 25 seconds to maintain streaming capabilities.

Packages

Package	Description
`edge-runtime`	Core runtime for local development and testing
`@edge-runtime/primitives`	Web API primitives (Request, Response, fetch, etc.)
`@edge-runtime/cookies`	Cookie parsing and serialization
`@edge-runtime/format`	Pretty-print Edge Runtime values
`@edge-runtime/vm`	VM-based Edge Runtime sandbox

Installation

npm install edge-runtime

# Individual packages
npm install @edge-runtime/cookies
npm install @edge-runtime/primitives

Local Development with Edge Runtime

import { EdgeRuntime } from 'edge-runtime'

const runtime = new EdgeRuntime()

const result = await runtime.evaluate(`
  const response = new Response('Hello from the edge!')
  response.text()
`)

console.log(result) // "Hello from the edge!"

Available Web APIs

The Edge Runtime provides a subset of standard Web APIs:

Fetch API: fetch, Request, Response, Headers
Streams: ReadableStream, WritableStream, TransformStream
Encoding: TextEncoder, TextDecoder
URL: URL, URLSearchParams, URLPattern
Crypto: crypto.subtle, crypto.getRandomValues
Timers: setTimeout, setInterval (limited)
Cache: CacheStorage, Cache
Structured Clone: structuredClone

Routing Middleware Pattern (Next.js 15 and earlier)

In Next.js 16+, middleware.ts is renamed to proxy.ts and runs on Node.js (not Edge). For Next.js 15 and earlier, or for Vercel Routing Middleware (non-Next.js):

// middleware.ts
import { NextRequest, NextResponse } from 'next/server'

export const config = {
  matcher: ['/api/:path*', '/dashboard/:path*'],
}

export default function middleware(request: NextRequest) {
  // Check auth
  const token = request.cookies.get('session')
  if (!token) {
    return NextResponse.redirect(new URL('/login', request.url))
  }

  // Add headers
  const response = NextResponse.next()
  response.headers.set('x-edge-powered', 'true')
  return response
}

Edge Function Pattern (Next.js)

// app/api/hello/route.ts
export const runtime = 'edge'

export async function GET(request: Request) {
  return new Response(JSON.stringify({ hello: 'world' }), {
    headers: { 'content-type': 'application/json' },
  })
}

@edge-runtime/cookies

import { ResponseCookies, RequestCookies } from '@edge-runtime/cookies'

// Parse request cookies
const cookies = new RequestCookies(request.headers)
const session = cookies.get('session')

// Set response cookies
const responseCookies = new ResponseCookies(response.headers)
responseCookies.set('theme', 'dark', {
  httpOnly: true,
  secure: true,
  sameSite: 'lax',
  maxAge: 60 * 60 * 24 * 7, // 1 week
})

Limitations

The Edge Runtime does not support:

Node.js built-in modules (fs, path, child_process, etc.)
Native/binary modules
eval() and new Function() (security restriction)
Long-running processes (execution time limits apply)
Full Node.js Buffer (use Uint8Array instead)

Key Points

Web Standards first — uses standard Web APIs, not Node.js APIs
Cold start < 1ms — no VM boot overhead at the edge (9x faster than traditional serverless)
Size limits — edge functions have a 1-4 MB size limit (varies by platform)
No file system — use fetch, KV, or external storage instead
Streaming supported — use ReadableStream for streaming responses (300s max duration, 25s to first byte)
export const runtime = 'edge' — opt into Edge Runtime in Next.js route handlers
Migration recommended — Vercel recommends Node.js runtime for most use cases; edge runtime is best only when ultra-low latency at the edge is critical

edge-runtime