workerd (pronounced “worker-dee”) is the JavaScript/Wasm runtime that powers Cloudflare Workers. It can be used as an application server to self-host Workers-compatible applications, as a development tool for local testing, or as a programmable HTTP proxy.
Key Features
- Worker-compatible runtime with Web-standard APIs such as
fetch(). - Support for JavaScript/TypeScript and WebAssembly, with build tooling based on Bazel and prebuilt binaries available via npm.
- Capability bindings and Cap’n Proto configuration model for composable and secure service wiring.
- Integration with Wrangler and Miniflare for local development workflows.
Use Cases
- Self-host Workers-compatible applications for compliance or offline scenarios.
- Run workerd as a programmable HTTP proxy or application server in place of traditional microservices.
- Use workerd in developer toolchains to iterate on edge code locally before deploying to Cloudflare.
Technical Highlights
- Single-threaded event loop model optimized for high-concurrency I/O.
- Web-standard APIs enable easy portability between browser/edge and server environments.
- Apache-2.0 licensed, active community, and frequent updates.