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Cloudflare Workers

Published: Jul 2021
Updated: Jul 2021

Cloudflare Workers provide an execution environment that allows you to create entirely new applications or augment existing ones without configuring or maintaining infrastructure. Basically, you can use Javascript to intercept and augment incoming requests before they hit your origin.

This guide covers building a deployment pipeline using the official wrangler. Wrangler is the official tool from Cloudflare to develop, build, and deploy workers. It has similar functionality serverless framework.

JS workers are structured differently than AWS Lambda functions (and perhaps other FaaS providers). AWS Lambda functions export a function handler. This makes them much easier to develop and tests. Unfortunately Cloudflare Workers execute in the global context. This requires a little shimming for automated testing.

Wrangler provides a “dev” environment. This transparently deploys the worker to Cloudflare’s dev cloud and proxies it back to localhost. This is your “F5 driven development” model. However, this has nothing to do with automated testing.

Wrangler also provides a publish command to deploy workers. It also supports an --env option for specific configurations in different environments.

Automated Testing

I recommend using jest for Javascript testing. Jest includes hooks for simulating Cloudflare’s execution environment. You’ll need to include node-fetch and use setupFilesAfterEnv to configure globals and anything else Cloudflare injects into the environment. My guess is that adding classes and functions around fetch is enough.

First, write the handler so it works with Node.js. This snippet checks for the CommonJS module for determining Cloudflare vs Node.js runtime.

async function handleRequest(request) {
  // do your stuff

if (typeof module === "undefined") {
  addEventListener("fetch", (event) => {
} else {
  module.exports = handleRequest;

Second, use Jests setupFilesAfterEnv to assign Request and Response globals and mock fetch.

// setup.jest.js

const { Request, Response } = require("node-fetch");

// see gotchas
Response.redirect = function (url, status = 302) {
  return new Response(null, {
    headers: {
      location: new URL(url).toString(),

// Assign whatever globals Cloudflare injects into the environment
global.Request = Request;
global.Response = Response;

// mock fetch so no network traffic ever occurs
beforeEach(() => {
  global.fetch = jest.fn();

Now you can write simple tests like this:

const worker = require("./index");

test("example", async () => {
  await worker(new Request());



Deployment’s are simple wrangler publish commands. I recommend introducing a specific production configuration into wrangler.toml. Cloudflare does not support multiple “environments” per-say. You can emulate that by changing the configured route listeners. Note that wrangler automatically appends the environment to the worker name in Cloudflare.

The pipeline should authenticate with environment variables.

Gotchas & Caveats

  • node-fetch version 2.6 does not include the Response.redirct function. This is included in version 3.0 which is currently in Beta at the time of this writing.
  • wrangler does not support multiple workers inside a single project. You’ll need to create multiple folders (with relevant package.json, wrangler.toml, etc) for all workers you’d like to keep in the same repository.
  • Cloudflare workers and wrangler are not competitors to serverless. These are apples to oranges. If you’ve worked with serverless you’ll find that workers + wrangler is a paired down setup.