Productivity in this case being in the eye of the beholder? I’d argue that people experienced in how node works, wouldn’t have to think too much, since async is the default. I agree with your general sentiment though.
If it is single threaded, and we are talking about shared variables, I thought I can assume the runtime is not going to pause my code execution, switch context, and run other code midway through my call back handler.
If we are talking about shared external resources (e.g. who can update a cloud blob) then we could have a proxy for that as a variable. You might need retry logic, and it could get tricky in that respect.
What I mean is that distinct from threading, where the following code could be interrupted between the first and second line of the function, by something that updates global:
var global
function addTheseToGlobal(a, b) {
im = a + global
return im + b
}