How to detect a Cloudflare Worker in dev local mode

When developing Cloudflare Workers, it's often necessary to differentiate between development and production environments to handle environment-specific logic, debugging, or configurations. A simple yet effective way to detect if your Cloudflare Worker is running in local development mode is by setting up environment variables.

This approach allows you to toggle between development and production modes seamlessly without hardcoding any values directly into your code. In this guide, I’ll show you how to implement a DEV flag using Wrangler, Cloudflare’s CLI tool, to easily distinguish between the two environments.

To detect if a Cloudflare Worker is running in local development mode, follow these steps:

Step 1: Define a DEV variable in wrangler.toml

In your wrangler.toml, set up a DEV variable that defaults to false:

DEV = false

Step 2: Override DEV in .dev.vars for local development

Create a .dev.vars file in your project's root directory, and override the DEV variable to true for local development:

DEV = true

This file will only be used when running in development mode.

Step 3: Access the DEV variable in your Worker

You can then access this DEV variable in your Cloudflare Worker through the environment configuration:

const { DEV } = env;

if (DEV) {
  // Logic for development
} else {
  // Logic for production
}

Step 4: Run in development

When you run the worker locally using wrangler dev, the DEV variable will be set to true, allowing you to detect the development environment. In production, the DEV variable remains false.