Question
I am currently reading Async Javascript by Trevor Burnham. This has been a great book so far.
He talks about this snippet and console.log being 'async' in the Safari and Chrome console. Unfortunately I can't replicate this. Here is the code:
var obj = {};
console.log(obj);
obj.foo = 'bar';
// my outcome: Object{}; 'bar';
// The book outcome: {foo:bar};
If this was async, I would anticipate the outcome to be the books outcome. console.log() is put in the event queue until all code is executed, then it is ran and it would have the bar property.
It appears though it is running synchronously.
Am I running this code wrong? Is console.log actually async?
Answer
console.log is not standardized, so the behavior is rather undefined, and
can be changed easily from release to release of the developer tools. Your
book is likely to be outdated, as might my answer soon.
To our code, it does not make any difference whether console.log is async or
not, it does not provide any kind of callback or so; and the values you pass
are always referenced and computed at the time you call the function.
We don't really know what happens then (OK, we could, since Firebug, Chrome Devtools and Opera Dragonfly are all open source). The console will need to store the logged values somewhere, and it will display them on the screen. The rendering will happen asynchronously for sure (being throttled to rate-limit updates), as will future interactions with the logged objects in the console (like expanding object properties).
So the console might either clone (serialize) the mutable objects that you did
log, or it will store references to them. The first one doesn't work well with
deep/large objects. Also, at least the initial rendering in the console will
probably show the "current" state of the object, i.e. the one when it got
logged - in your example you see Object {}.
However, when you expand the object to inspect its properties further, it is
likely that the console will have only stored a reference to your object and
its properties, and displaying them now will then show their current (already
mutated) state. If you click on the +, you should be able to see the bar
property in your example.
Here's a screenshot that was posted in the bug report to explain their "fix":
So, some values might be referenced long after they have been logged, and the evaluation of these is rather lazy ("when needed"). The most famous example of this discrepancy is handled in the question [Is Chrome's JavaScript console lazy about evaluating arrays?](https://stackoverflow.com/questions/4057440/is- chromes-javascript-console-lazy-about-evaluating-arrays)
A workaround is to make sure to log serialized snapshots of your objects
always, e.g. by doing console.log(JSON.stringify(obj)). This will work for
non-circular and rather small objects only, though. See also How can I change
the default behavior of console.log in
Safari?.
The better solution is to use breakpoints for debugging, where the execution completely stops and you can inspect the current values at each point. Use logging only with serialisable and immutable data.
