Serverless Functions Explained: What They Are and When They Make Sense

The name 'serverless' is terrible marketing. There are definitely servers. The difference is you don't think about them: you write a function, deploy it, and someone else handles provisioning, scaling, and maintenance.

What a Serverless Function Actually Is

A serverless function is a piece of code that runs in response to an event (an HTTP request, a file upload, a scheduled time). It runs, does its work, and stops. The cloud provider handles starting new instances when requests come in and stopping them when they're done. You write the function. They handle everything else.

Where Serverless Shines

• API endpoints with unpredictable or spiky traffic
• Background jobs triggered by events (email sending, image processing after upload)
• Scheduled tasks (cron jobs) without maintaining a server
• Webhooks from third-party services
• Authentication handlers and lightweight data validation
• Supplementing a traditional backend for specific high-traffic endpoints

The Practical Reality of Cold Starts

Cold starts are the most-discussed serverless limitation. If a function hasn't been called recently, the provider spins up a new environment, which takes time. For a user clicking a button, a 500ms cold start is noticeable. For a background job nobody's waiting on, it doesn't matter. Edge runtimes (Vercel Edge, Cloudflare Workers) have near-zero cold starts because they run in JavaScript environments already warm at edge locations.

Vercel and Next.js Serverless

If you're using Next.js on Vercel, you're already using serverless functions for API routes and server components. Each API route becomes a serverless function automatically. Route handlers (app/api/route.ts) run as Edge Functions by default in some configurations, or as serverless Node.js functions. For anything that needs to be fast globally, add export const runtime = 'edge' to use the Edge runtime with near-zero cold starts.