Sunder is a framework for Service Workers like those found in Cloudflare Workers. It allows you to create testable APIs and websites in a similar way as you would using Node.js's Koa or Express framework.
Sunder is designed to be minimal, testable, and easy to understand. It's only a few hundred lines of code.
Perhaps read the Introductory Blog Post, or click the links in the side bar to get started.