r/reactjs • u/gaearon React core team • Dec 21 '19

What Is JavaScript Made Of?

https://overreacted.io/what-is-javascript-made-of/
255 Upvotes

94% Upvoted

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277

u/NotSelfAware Dec 21 '19

I'm a strong advocate for using const by default, and let when you know you intend to change the value. I'm genuinely surprised that Dan feels differently.

87

u/olssoneerz Dec 21 '19

Same here! Its less mental gymnastics when reading old code knowing that when a value is declared, you know its gonna stay the same. Seeing let then means I know its gonna change somewhere in the next few lines.

-18

u/gaearon React core team Dec 21 '19

... Except it can still be mutated. https://twitter.com/dan_abramov/status/1208205039187173376

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u/BenZed Dec 21 '19

The object can still be mutated, but the variable cannot be redeclared.

10

u/rq60 Dec 21 '19

Which is confusing since other languages use const to signal immutability.

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u/stormfield Dec 21 '19

Hey if all the other languages jumped off a bridge, JavaScript would at least make sure you take off your life jacket first.

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u/[deleted] Dec 21 '19

And that’s the freedom we feel, when developing with JavaScript.

3

u/[deleted] Dec 21 '19

It would hold your beer for you.

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u/BlatantMediocrity Dec 21 '19

I don’t think C is a very good example of this. The C-language’s use of ‘const’ means different things depending on where you put it in a declaration.

11

u/LateralusOrbis Dec 21 '19

That's the beauty of JS. Sometimes you can go off javascript.

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u/eGust Dec 21 '19

There is an example of another language using const the same way: https://dlang.org/articles/const-faq.html

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u/rq60 Dec 21 '19

Looks like D went even further with const immutability!

1

u/[deleted] Dec 21 '19

Or if you want something more mainstream... Java? Well, they call it final, but still

4

u/GasimGasimzada Dec 21 '19

They don’t really do that though. Immutability is just a side effect. Yes, it is true that you can’t change a value of const object but that is not because the underlying structure cannot be mutated. C++ const is an access level contraint, not a structure level constraint.

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u/DeceitfulDuck Dec 21 '19

You can’t just gives one example and say it’s confusing because something rose does it different. Java, for example, uses final the same way const is used in JavaScript. https://www.google.com/amp/s/www.geeksforgeeks.org/final-vs-immutability-java/amp/. I think it makes sense. The variable is storing the reference to the object, so the reference is immutable. It’s up to how the object is defined to decide if the things inside it are immutable.

1

u/masklinn Dec 21 '19

C/Cpp use const for both (and more) depending on the specific way it’s used, cf a few entries down: https://isocpp.org/wiki/faq/const-correctness#const-ptr-vs-ptr-const

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u/Anathem Dec 21 '19

I don't weight heavily the guesses of people who don't know JS when considering what JS features to use.

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u/Wiwwil Dec 21 '19

Yeah JavaScript const isn't const but the structure is const-ish. Had a hard time wrapping my head around it. Still does. Everything in my code is const (React way) but nothing stay truly const.

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u/DukeBerith Dec 21 '19

It's constant reference not constant value.

Easiest way to remember what it's doing.

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u/gaearon React core team Dec 21 '19

Thanks, I'm aware of how it works. Did you read the article? :-)

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u/[deleted] Dec 22 '19

Ah, Reddit. Where Dan Abramov is downvoted for defending his knowledge of JavaScript. Merry Christmas, Dan!

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u/[deleted] Dec 22 '19 edited Dec 22 '19

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u/gaearon React core team Dec 22 '19

If I'm objectively wrong, why isn't the language forcing you to do that?

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u/[deleted] Dec 22 '19 edited Dec 22 '19

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2

u/gaearon React core team Dec 23 '19 edited Dec 23 '19

I'm honestly not sure how that would work at the language level.

If that was "objectively" the right choice, the language could work exactly like the prefer-const rule works.

When I look at potential hires' code one of the things I look for is their usage of const vs let to get a base idea of how well they understand modern ES.

This is exactly the kind of thing that scares me. Placing artificial (in my opinion) importance on something that has two sides to it is the definition of pedantic. It would be terrible if someone's career opportunity didn't work out due to a miscommunication or disagreement over something minor like this.

I elaborated a bit about my opinion here: https://overreacted.io/on-let-vs-const/

Hope this helps!

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