The Edge Runtime executes JavaScript on V8 isolates without Node.js. That makes it fast to cold-start, but you can't use Node-only modules.
What works
Web Standards APIs: fetch, Request, Response, URL, crypto.subtle, TextEncoder, and most of WHATWG Streams.
What doesn't
Anything that depends on the Node runtime: fs, path, process.binding, native add-ons. Most popular Node libraries have edge-friendly forks; check before pinning the runtime.