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Once and for all: const in JavaScript is not immutable

Valentino Gagliardi on July 12, 2019

I know that's been said hundred of times, but I still see people proclaiming (even in some JavaScript books) that const is immutable. It is not. ...

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Saurabh Sharma • Jul 12 '19

The thing is const never let you to change the memory address it refers to.

you but can mutate the value itself.

Mike • Jul 12 '19

Const makes the reference immutable, and for primitives that means the value is immutable too.

But for Objects, the difference matters.

Valentino Gagliardi • Jul 12 '19

yes!

Prahlad Yeri • Jul 12 '19

That's just because of the nature of JavaScript, that's how it is.

In JavaScript, some built-in types (numbers, strings) are immutable, but custom objects are generally mutable.

So, it matters not whether you've declared them as const or not, what matters is whether they are custom objects (like in your example) or simple numbers/strings. The latter are obviously immutable whether you use const or not.

And even with your custom object, you can make individual attributes immutable like this (again, const doesn't matter here):

Object.defineProperty(person, 'name', { value: 'John', writable: false });

So when you do this, it'll be silently ignored since the person object is now immutable:

person.name = "Valentino";

This is irrespective of whether you defined person as let, const or var!

Valentino Gagliardi • Jul 12 '19

My point is that there are people out there still saying "const is intrinsically immutable". While it's not. As for objects >> Protecting objects from manipulation

Nicholas Napoli • Jul 14 '19 • Edited

I would argue that const is intrinsically immutable. Posing the argument that making a reference immutable, but the data inside is not, is not relevant. Granted. It would be nice if this were default behavior, but knowing why the data inside is not const is important.

This flows down to the engine that js is running on in lower level language land, and that land explains this concept of why well. It should not be a debate.

George • Jul 12 '19

If you want immutability then you need to start using Object.freeze() and Object.seal()

Valentino Gagliardi • Jul 12 '19

Object.freeze is shallow though :-)

Saurabh Sharma • Jul 12 '19 • Edited

So that's why there libraries like Immutable.js, but it would be great if JS has something for making values immutable.

Kelly Brown • Jul 12 '19

It's all a war of semantics. Technically, the variable itself is immutable for its lifetime; it just so happens to act as a gateway to data that is quite mutable. This is why I hate the way Java tries to teach students: "Everything is pass by value." Yeah, everything is pass by value... but the value happens to be a reference to something else. Strange that something passed by "value" lets me trigger side effects for observers of that data.

Good post.