What happens when when assigning values to a variable outside a callback in node.js?

Question

Given a set up like the following:

var foo = -1;

db.myCollection.findOne({_id : 1}, function(err, doc) {
    foo = doc.bar; //bar == 99 is true

});

console.log(foo);

I have found that the the console always outputs 99. Why does the code wait? Isn't this not asynchronous?

If `findOne` were asynchronous, that code **wouldn't** print 99. It'd print -1. So if you're seeing 99, then clearly `findOne` is synchronous. — T.J. Crowder, Nov 01 '16 at 16:52
I don't know how we should know anything about the code you're running. If the behavior is synchronous, then it's synchronous. If not, then it's not. Have you consulted the documentation? — , Nov 01 '16 at 16:57
Looking at the question, you do understand the concept. With no indication of what the `.findOne()` function actually does, how should we know anything beyond what you provided? Maybe this is yet another an example that doesn't actually represent the problem. It sure seems that way. After all, a method named `.findOne()` *suggests* that it performs a search for *one* item, which means a callback would make little sense if it was synchronous. But then AFAIK `mongodb` has a `db.collection.findOne()` that takes a callback and *is* async... — , Nov 05 '16 at 14:14
So the truth is that I *was* reading between the lines, and nothing added up. All we have to rely on is your observance as presented, and again, if it is synchronous, then it is. If not, then it's not, and consulting the documentation would be the first step to take if uncertain. Either way, if you ask a vague question that doesn't make much sense, you'll be challenged on it. — , Nov 05 '16 at 14:15

T.J. Crowder · Accepted Answer · 2016-11-01T17:01:54.117

Not all callbacks are asynchronous. It depends on the mechanics of the method you're passing the callback to.

For instance, the sort method on arrays takes a callback that it calls synchronously; sort doesn't return until it's made all of its calls to the callback you give it:

var a = [2, 8, -1];
console.log("before");
a.sort(function(x, y) {
  console.log("callback called with", x, y);
  return x- y;
});
console.log("sort complete:", a);

That outputs:

before
callback called with 2 8
callback called with 8 -1
callback called with 2 -1
sort complete: [-1, 2, 8]

(Note that the exact order and number of the "callback called with" lines will vary depending on your browser's JavaScript engine.)

In contrast, the fs.open call in NodeJS's API calls its callback asynchronously; it returns after starting the process of opening the file, but doesn't wait for that process to finish, and then it calls the callback when the process finishes later. I can't provide an on-site example of fs.open, but setTimeout also calls its callback asynchronously:

console.log("before");
setTimeout(function() {
  console.log("in the callback");
}, 0);
console.log("after");

That outputs:

before
after
in the callback

What happens when when assigning values to a variable outside a callback in node.js?

1 Answers1