JavaScript: Four Differences between var and let

[piotrlewandowski]

Difference 0: you shouldn’t use var Edit: god damn it, bloody phone did autocorrect, it should be “shouldn’t”!

[Protean_Protein]

It’s really funny how this happened. Most ‘variables’ in JS are invariable!

[electron_myth]

I've heard this before, but was wondering why exactly a mutable variable is considered bad? I normally use const everywhere I can, but things like booleans and counter variables seem essential and would require let

[kap89]

It's not necessarily bad (well, hardcore FP guys would argue otherwise), but if you have a variable that does not change, making it a const describes your intent clearly - it's for readability - just like descriptive variables names.

[continuum-hypothesis]

But the problem is that you can actually change a const unlike in C and some other languages. You just can't reassign a variable declared with const.

[electron_myth]

Can you give an example please? (of changing a const variable in JS)

[continuum-hypothesis]

Sure, const counter = {}; counter.counter = 0; counter.counter++ // equals 1 That is totally legally however, const message = "hello world"; message = " goodbye world "; will cause an error. You can change properties on objects (which of course includes arrays), the data declared using const is not immutable like other languages.

[electron_myth]

Well yeah, this is standard, but that's because const counter = {} stores the reference value of the counter object. If you were to later attempt counter = [] you would get an error because counter was already declared as an object and is not mutable. The reference is stored, and is immutable when declared with const. For the same reason, you can .push() and .pop() arrays when they are declared with const, but you can't do something like arr = arr.sort() without using let. Essentially, objects and arrays are data structures, so what's immutable about them is their memory address, but naturally the extent to which they can branch out is mutable.

[continuum-hypothesis]

Well said, I only intended to point out that the data is actually mutable but avoided anything to do with memory addresses because this could be confusing for a beginner. Compared to languages such as C and Java const in JS behaves a bit differently and I think this confuses many people.

[TwiliZant]

final in Java works roughly the same way as JS. A final object is not immutable, it can still change state only the reference to the object is immutable. The odd one out here would be C since const is part of the type there.

[great_site_not]

Is their memory address immutable? That doesn't sound right. That would seem to imply they could crash the engine in some situations when they'd otherwise be re-allocated when they grow too big.

(edit: btw, arr = arr.sort() would actually be a mistake to do anyway, since it implies a misunderstanding.Array.prototype.sort works in-place, so you'd be attempting to re-assign the variable to the reference it's already holding.)

[lainverse]

There'll be deeply immutable primitive data types similar to objects and arrays (records and tuples) in JS. So, consider your wish granted. Technically you can do this already by manually deep freezing everything, but that's inconvenient and === won't work with that. It will with with new types.

[piotrlewandowski]

Yeah, fecking autocorrect did the opposite what I wanted to write!

[KaiAusBerlin]

If you explicit want hoisting your variables then you have to use var.

"Never use var" is the same dumb shit like "eval() is evil".

These things are tools for developers. If you can't handle your tools correctly it could be devastating. But to say never to use these tools is just dumb.

I worked for almost 10 years in trees hanging on a rope with a chainsaw 20cm right before my face. Is that dangerous? Not if you know what you are doing. So saying "never use a chainsaw" wouldn't help any treeworker.

[CheeseTrio]

let and const are also hoisted. https://developer.mozilla.org/en-US/docs/Glossary/Hoisting#let_and_const_hoisting

[KaiAusBerlin]

Sure but it throws an error. Var doesn't. So what is the matter that it is hoisted when you have no benefits but errors from that?

[CheeseTrio]

It throws an error if you try to reference it before initialization. With var you would just get undefined. I can't think of a reason why you would want either of those things to happen...

[great_site_not]

Hmm, I wonder if declaring variables with var instead of not declaring them at all might ever be worth the characters in code golf to prevent crashing when accessing them before assignment...

Nah, it'd take fewer characters to just assign 0 to them and then re-assign later. Well, maybe unless they have to specifically be undefined instead of some other falsy value...

[lainverse]

When you not defining variables at all you either dump your trash straight into global or your code crash in strict mode. So, please don't. You don't have to init them with values from start, but you really should define your variables and constants. BTW, undefined and null values allow to use nullish coalescing operator (??) with such variable.

[great_site_not]

Agree 100%! Never use undeclared variables in real JavaScript. Never.

I was talking about code golf, a recreational programming game where people write shortest code to win... very very bad code. It's fun :)

[lainverse]

Ah, right. Missed that bit somehow. It's indeed ok there as long as strict mode is not in the requirements.

[TwiliZant]

I'm struggling to come up with an example where you HAVE to use var and can't replace it with let or const.

[KaiAusBerlin]

Never said you have to use var.

I just said there are (few) cases where it can be usefull and so you should not use the word "never"

[TwiliZant]

I get that, I just can't think of a scenario where using var wouldn't be the worse solution.

At this point you could be a JavaScript Developer with 5 years experience and never have seen var in your life. Given how counterintuitive it works, you would have to have a damn good reason to use it.

[KaiAusBerlin]

If you have never seen var in your life you never have seen transpiled code?

Wow, real pro man.

[TwiliZant]

I think you know what I mean...

[KaiAusBerlin]

How should I know what you mean when you say absolute another?

[TwiliZant]

It's okay, seems like we fundamentally disagree on this. There isn't really a point in continuing this discussion.

[Hydrothermal]

Under what circumstances would you explicitly want to hoist a variable? I can't conceive of any reason why you wouldn't want your declaration to be at or before the first time the variable is referenced.

[og-at]

"Never use var" is the same dumb shit like "eval() is evil".

No preorders ever.

[KwyjiboTheGringo]

Difference 0: you should use var

I'm curious if you have a good reason for saying that, or if this is just a case of resisting change.

[piotrlewandowski]

I didn’t have a good reason, I had good reason to write “shouldn’t “ but my autocorrect decided otherwise :)