If you want to know JS deeply, I think reading the EcmaScript language specification is a very good step. I've read through it before; it's quite long but not overly difficult reading.
Studying the source code for a JS implementation is also a good step. QuickJS is a good one.
Build things, get stuck, read the docs, build more things, repeat.
anything by kyle simpson
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For JS in general: anything from https://2ality.com - articles, books, walk-throughs, etc. Axel is thorough and systemically reconstructs complex topics from core building blocks.
For Cloudflare Workers specifically: first, learn regular Web Workers, then read everything from kentonv who's the tech lead for CF Workers, and a user here. He's written many (disjointed) pieces on Workers both here on HN and on the CF community board. Additionally, Cloudflare Developers Discord (https://discord.com/invite/cloudflaredev) has a very active channel for Workers with people discussing implementation details and edge cases.