Very nice! There are a few things:

• https://propelml.org/notebook?nbId=tWdWpdiM1o3uixg4sQRb -> when I apply this PR, this notebook throws an error in the console after 2 seconds.
• The initial page uses the node-based sandbox, however when I refresh the page it uses the iframe sandbox again.
• The node worker process isn't actually sandboxed. Would it be possible to use the vm module to actually make it secure. I do agree with creating a separate process for every sandbox, please keep that.
• Is it possible to not use the same server for the sandbox and the web server. E.g. the website is served from localhost:8080, and the sandbox is on localhost:8081.
• Use the iframe sandbox by default, and add a button "connect to local sandbox" if there is a server running there. (this may be out of scope for this PR, we could do it later. In that case it would be nice to pass the address of the local sandbox as part of the url, e.g. http://localhost/#notebook/anonymous?vm=8081.

@piscisaureus Thanks for the review, I've fixed the errors (the reason was that TextDecoder is not present in Node.js, and it is used inside fetchString() in sandbox.ts), and I'm using VM module now.
I prefer to make this patch small and I won't include any UI related stuff right now and will do that in a follow-up pr : )

I prefer to make this patch small and I won't include any UI related stuff right now and will do that in a follow-up pr : )
That's ok - but then can you make it so that I can connect to the local sandbox by URL and stay on that URL (so it doesn't use the web sandbox after a page reload)?

@piscisaureus Now you can use node-based sandbox by passing server address in your url, like:
http://localhost:8081/#/notebook/anonymous?ws=ws://localhost:8081