Ok, thanks, I get it now. In my case a macro transforms the body of each callback to catch exceptions and pass them back as error args through the callback chain. So in your example, there would be a generated try-catch around the JSON.parse(data). I forgot this detail (sign of a successful abstraction?) and it does seem an example of something macros can do that functions cannot.

Re null reference checks, to get behavior analogous to a null exception you have not only to check for null, but also pass back an explicit error if you find it. That's a lot more work than adding in an extra question mark. Null checks that do nothing but not crash are a mixed blessing; 90+% of the time they do what you want, but when they don't, you get a silent failure and a debugging goose chase. I'd be surprised if you told me that that never happens.

I took a look at Node.js domains and they do seem really complicated. If I were working in Javascript instead of having control over the language, I doubt I would use them; I would probably just crash-and-restart as one of the other commenters described. That's not a good solution, but probably the best tradeoff given the alternatives.