Cloudflare Workers: your first serverless function

By Flavio Copes

What Cloudflare Workers are and how to build, run, and deploy your first one. The foundation of the whole Cloudflare developer platform.

I run a few things in production on Cloudflare, and it all starts with the same building block: a Worker.

A Worker is a small piece of JavaScript that runs on Cloudflare’s servers. There’s no server to set up, no machine to keep alive. You write a function, deploy it, and it runs.

The neat part is where it runs. Cloudflare has servers all over the world, and your Worker runs on the one closest to whoever is calling it. So it’s fast for everyone.

This is the first post in a series where I document the Cloudflare tools I actually use. Everything else, the databases, storage, queues, builds on top of Workers. So let’s start here.

What a Worker looks like

A Worker is a function that takes a request and returns a response.

export default {
  async fetch(request, env, ctx) {
    return new Response('Hello from the edge!')
  },
}

That’s a complete Worker. When someone hits its URL, the fetch function runs and returns the response.

The three arguments matter:

• request is the incoming request, a standard Request object.
• env holds your bindings: databases, storage, secrets. More on those in later posts.
• ctx lets you do work after the response is sent, with ctx.waitUntil().

If you’ve used the Fetch API in the browser, this is the same Request and Response. No new framework to learn.

Create a project

The fastest way to start is the official scaffolder:

npm create cloudflare@latest my-worker

Pick the “Hello World” Worker when it asks. It sets up the project, installs wrangler (Cloudflare’s CLI), and creates a wrangler.jsonc config file.

Then move into the folder:

cd my-worker

Run it locally

You don’t deploy to test. Cloudflare runs the same engine on your machine:

npx wrangler dev

This starts a local server, usually on http://localhost:8787. Open it and you’ll see your message. Edit the code, save, and it reloads.

Read the request

Let’s do something with the request. Here we read the path and respond based on it:

export default {
  async fetch(request, env, ctx) {
    const url = new URL(request.url)

    if (url.pathname === '/') {
      return new Response('Home')
    }

    return new Response('Not found', { status: 404 })
  },
}

You can return JSON just as easily:

return Response.json({ ok: true })

Deploy it

When you’re happy, one command puts it live:

npx wrangler deploy

Wrangler uploads your Worker and gives you a URL like my-worker.your-name.workers.dev. That’s it, your code is running worldwide.

The first time, it’ll ask you to log in to your Cloudflare account in the browser.

What comes next

On its own, a Worker is just a function. The power comes from bindings: you attach a database, a storage bucket, a queue, and they show up on that env argument.

That’s what the rest of this series is about. A database with D1, files with R2, key-value with KV, background jobs with Queues, and more.

But it all starts here, with a function that returns a response.

If you want to read ahead, the Workers docs are excellent.