Mutable States can change inside your Browser console log

So we know, that web development must be one of the fastest-changing ecospheres humankind has ever seen (not to say, JavaScript frameworks and their best practices definitely mutate similar in frequency and deadliness as Coronaviruses). While these new developments can also come with great joy and many opportunities, this means that once in a while, we need to take care of older projects which were written in a completely different mindset.

It’s somehow trivial: Even when your infrastructure is prone to constant shifts, any Software Developer holding at least some reputation should strive to write their code as long-living and maintainable as originally intended. Or longer.

But once in a while you run into legacy code that you first have to dissect in order to understand their working. And for JS, this usually means inserting console.log() statements at various places and to trace them during execution (yeah, I know, there’s a plenitude of articles telling you to stop that, but let’s just stay at the most basic level here).

Especially in an architecture with distributed, possibly asynchronous events (which helps in reducing coupling, see e.g. Mediator and Publish-Subscribe patterns), this can help your bugtracing. But there’s a catch. One which took me some time to actually understand as quite the villain.

It does not make any sense to me, but for some reason, at least Chrome and Firefox in their current implementation save some effort when using console.log() for object entities. As in, they seem to just hold a reference for lazy evaluation. It can then be that you look upwards at your log, maybe even need to scroll there, look at some value and then not realize that you are looking at the current state, not the state at time of logging!

Maybe that was clear to you. Maybe it never occured to you because you always cared about using your state immutably. But in case you are developing on some legacy code and don’t know about what your predecessor did everywhere, you might not be prepared.

You can visualize that difference easily by yourself. Consider that short JS script:

var trustfulObject = {number: 0};
var deceptiveObject = {number: 0};

// let's just increase these numbers once each second
setInterval(() => {
    console.log("let's see...", trustfulObject, deceptiveObject);
    trustfulObject = {number: trustfulObject.number + 1};
    deceptiveObject.number = deceptiveObject.number + 1;
}, 1000);

Let that code run for a while and then open your Browser console. Scroll upwards a bit and click on some of the objects. You will find that the trustfulObject is always enumerated as supposed (at the time of logging), while the deceptiveObject will always show the number at the time of clicking. That surely surprised me.

In case you are still wondering why: The trustfulObject is freshly created each step and then reassigned to your reference variable. It seems the Browser has no other choice than logging the old (correct) state, because the reference is lost afterwards. The deceptiveObject holds the same reference during the whole runtime, which somehow makes it look more efficient to the Browser to just not evaluate anything until you want to know the value.

And then, it lies to you. ¯\_(ツ)_/¯

Two notes:

  1. If you really have to deal with legacy code of a given size where you cannot easily change that behaviour, you can log your object using JSON.stringify, i.e. console.log("let's see…", trustfulObject, JSON.stringify(deceptiveObject)); avoids that lazy evaluation.
  2. Note: Not to be confused, the JS “const” keyword does exactly the opposite of creating an immutable object. It creates an immutable reference, i.e. you can only manipulate their content afterwards. Exactly what you not want.

Of course, in modern times you probably wouldn’t write vanilla JS, and e.g. using React useState definitely reduces that issue. But still. If you don’t want to use React & Co. everywhere, then… pay attention.

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