Worker threads are share-nothing, closer to processes than actual threads.
Worker threads aren't share-nothing, you can share data through SharedArrayBuffer which lets you share arbitrary binary data.
If they were share nothing, we probably wouldn't have shipped them.
Source: I'm Node.js core and one of the people who worked on them
You can also share arbitrary binary data between processes by using shared memory (e.g. Python has support for this in multiprocessing.shared_memory module).
Sorry, I realized after posting that I should have said "share-nothing by default". There are ways to share memory with a worker, but it's explicit and opt-in via a dedicated API. It's not like in Python where you can arbitrarily change global state from any thread.