I spent some time with one of the Caja developers back in 2010 or so, before it was made public. From memory, the problem they were trying to solve was a bit d

I spent some time with one of the Caja developers back in 2010 or so, before it was made public. From memory, the problem they were trying to solve was a bit different. From what I remember, they wanted to be able to run potentially hostile user supplied javascript code inside the JS VM purely using source code level validation. So for example, Caja needed to make sure the sandbox container didn't access the global object (since then it could escape its sandbox). And because simple code like this: (function () { return this })() evaluates to the global object, they banned the keyword this in sandboxed code. I'm hoping there's a way we can give untrusted code more or less full access to the JS environment, but just limit its access to the rest of the operating system. Javascript was first developed for web browsers, and to this day most javascript still has little to no need to access the rest of the operating system directly. But Javascript's obsessively granular modularity works in our favor here. If you look at a library like express, the core library makes vanishingly few calls to the nodejs environment. `app.listen()` is the only method I know about which wouldn't "just work" in this new world I'm proposing. And thats just a convenience wrapper around `require('http').createServer(app)` anyway. All the hard work happens in libraries like express.static - but thats trivially easy to swap out for another package that supports capabilities correctly, if we need to do that. A bad library could always be buggy - we can't stop that. I mostly want to stop opportunistic developers from taking advantage of the machines their modules are run on, so we can detect (and stop them) from doing nasty things. But as a few people have mentioned, this approach might be stuck "always playing defense". The nice thing about caja is that it was "complete". There were no weird edge cases left over in the language that the sandbox authors didn't consider. Thats what I'm worried the most about here.