What is the difference between the Edge and Node.js runtimes?

Question

Accepted Answer

Next.js can run server code in two runtimes. The **Node.js runtime** (default) is the full Node environment; the **Edge runtime** is a lightweight, V8-based environment that runs in globally-distributed edge locations close to users.

## Selecting a runtime

```ts
// per route handler / page
export const runtime = "edge";   // or "nodejs" (the default)
```

## The trade-offs

```text
                  Node.js runtime         Edge runtime
────────────────────────────────────────────────────────────────
APIs              full Node (fs, crypto,  Web APIs only (fetch,
                  Buffer, native modules) Request/Response) — limited
npm packages      all                     only those without Node APIs
Cold start        slower, heavier         near-zero, lightweight
Location          regional (one region)   global (near the user)
Latency to user   higher if far away      very low (geo-distributed)
Execution limits  longer, more memory     short CPU time, less memory
Database          direct TCP connections  HTTP-based DBs only (no raw TCP)
```

## When to use Edge

```text
✓ Middleware (auth checks, redirects, geo-routing) — must be fast & global
✓ Simple, latency-sensitive endpoints (personalization, A/B, feature flags)
✓ Reading geolocation/headers to tailor responses
```

Edge shines for **lightweight logic that benefits from being close to the user** and from instant cold starts.

## When to use Node.js

```text
✓ Direct database connections (Prisma over TCP, etc.)
✓ Heavy computation, large dependencies, native modules
✓ Anything using Node-specific APIs (fs, crypto.randomBytes via Node, Buffer)
```

## A real constraint example

```ts
export const runtime = "edge";
import fs from "fs";           // ❌ not available on Edge — build/runtime error
import { PrismaClient } from "@prisma/client"; // ⚠️ needs HTTP/data-proxy on Edge
```

Many popular libraries (including ORMs that open raw DB sockets) don't work on Edge without an HTTP-based adapter — a common gotcha.

## Why it matters

Choosing the runtime is a real architectural decision: **Edge** for fast, global, lightweight logic (especially middleware) with minimal cold starts, **Node.js** for full API access, direct DB connections, and heavy work.

Knowing Edge's limitations (Web APIs only, no raw TCP, short execution) prevents the frequent mistake of putting database-heavy or Node-dependent code on a runtime that can't support it.