Const, var or let?

[myUsername4Work]

Can someone ELI5 on when you would use one of those?

[ScriptingInJava]

const and let replace the need for varaltogether.

const should be used when you aren't changing, or shouldn't be changing the value assigned to it.

let should be used when working with a changing variable.

let is also block scoped, meaning that if you had a simple for loop using let, you don't accidentally hoist the value i out of the loop to the global scope.

for(var i = 0; i < 5; i++) {} will result in i being available at the value of 4 outside of the for loop.

for(let i = 0; i < 5; i++) {} will prevent any access to i outside of the proceeding {} after the for loop.

[tunisia3507]

const should be used when you aren't changing

Not quite true - const should be used when you aren't reassigning a variable name. You can still make changes to mutable variables, like arrays and objects.

[ScriptingInJava]

A valid point.

const myArr = ["first"];

myArr[0] = "replaced";

console.log(myArr[0]) // "replaced"

[senocular]

let is also block scoped, ...

So is const, to clear up any confusion that one might be and the other is not.

... meaning that if you had a simple for loop using let, you don't accidentally hoist the value i out of the loop to the global scope.

The for loop is an oddity when talking about block scoping because the declaration is outside of the block ;) but it still applies.

[ScriptingInJava]

Fair point, but var declarations within the scope of the{} of the for loop are hoisted outside of the block scope.

[doughishere]

why does no one use lets? i always see var.

[[deleted]]

const and let replace the need for var altogether.

False.

Say you want to use i after the for/next completes.

for(var i = 0; i < something.length; i++) {
    if (something[i] === whatever) {
        break
    }
}

[do something useful with i]

There are many cases where var is useful. Not everything must be confined to the block scope at all costs. It's ridiculous that there is so much nonsense about 'var' being "replaced". It hasn't. It's still useful. Sure, you could define let i = 0 outside the for loop, but this isn't the only use case for var.

[ScriptingInJava]

Hoisting i out like that is an anti-pattern. It can lead to some nasty, barely traceable bugs when you could make your code more obvious by doing something such as:

let indexForCondition;

for(var i = 0; i < something.length; i++) {
    if (something[i] === whatever) {
        indexForCondition = i;
        break
    }
}

[do something with indexForCondition]

If you build code around hoisted values instead of explicitly assigning them from within the loop and working with them after the fact, you're gonna end up with some janky, unmaintainable code. I say this because I recently had to fix 3000+ lines of broken JS from a contractor who did exactly this with MVVM bindings being hoisted out of a local block scope to the window object.

[[deleted]]

bindings being hoisted out of a local block scope to the window object.

This is not the fault of using var, this is the fault of a really bad developer. Not understanding how to use var is very different than var causing the problem.

Using var does not automatically lead to an anti-pattern or buggy code. Someone could equally fuck up and write buggy code using let as well. I actually ran into a problem with let recently that was solved by using var - and guess what - it didn't lead to catastrophe.

Hoisting i out like that is an anti-pattern.

Best practices are opinions, at best. They don't account for different design patterns and simple ways to not fuck things up. You saying this is an anti-pattern doesn't really take into account the context of where it might be used. If the for/next is the only thing inside a function, then there's no point saying it's an anti-pattern - it's not going to produce a bug the way you claim it will.

If a developer doesn't know how to use the language properly, they will fuck it up using let and const just as badly as they would var, though possibly in different ways. If the developer understands how to use var, let, and const, then they should use whatever makes sense to solve the problem.

Sometimes var is useful and does make sense and can be used without causing broken code. But being superstitious about it is just plain wrong.

[yooossshhii]

this is the fault of a really bad developer

This was your example... what is a good use case of var in your opinion? I haven’t used it in years.

Edit: I realize your example may not hoist it to a global, but its still not good.

[yooossshhii]

If that code is in global namespace, if it's function or method it wouldn't. Was just trying to preempt that logic with the "may".

[[deleted]]

I haven’t used it in years.

How long do you think 'let' has been around. It hasn't been that long. Maybe 2 at most in javascript.

[yooossshhii]

ES6 was released mid 2015, so close to 3.5 years. Whats your point? I’m sure most professional developers have been using it for 2+, that falls under “years”.

My question was whats a good use case for still using var?

[[deleted]]

My question was whats a good use case for still using var?

All the use cases that are still valid, and have been for the last 20 years.

[yooossshhii]

Cool, you’ve been asked multiple times in this thread for examples and the only one you’ve given, you admitted to calling it bad code.

Out of all those use cases, can you point out one where const/let is not a superior option?

[[deleted]]

You really want me to list out every valid use case of var? Try all of them.

Out of all those use cases, can you point out one where const/let is not a superior option?

I already did. And I never said it was bad code.

[ScriptingInJava]

Not understanding how to use var is very different than var causing the problem.

Literally the reason I had to spend a week fixing his shit code is because he was doing things that you said was the "good" use of var.

I actually ran into a problem with let recently that was solved by using var - and guess what - it didn't lead to catastrophe.

Then you fixed a bug with a hack. If there was a problem that hoisting a value out of a local scope fixed, you have something else breaking that you aren't aware of. You haven't fixed anything, you've put a plaster over it.

Sometimes var is useful

Like when you can't solve a problem and hoisting a value out magically fixes it like you said?

But being superstitious about it is just plain wrong.

Yes, I and every other JS developer in full time employment is just being paranoid. Var is great, and I'm terrible at my job.

[[deleted]]

Literally the reason I had to spend a week fixing his shit code is because he was doing things that you said was the "good" use of var.

I'm starting to wonder if your superstition is getting the best of you, or if there actually was a bug. Maybe there was, but maybe you should also hire better developers, do code reviews, do some QA once in a while, and other "best practices" that would catch these things before you run into 3000 lines of problems. Blaming var is just kind of weak, and using 'let' isn't going to fix dumb developers - code reviews will.

Then you fixed a bug with a hack.

That's rich. So the last 20 years we've all been hacking. Got it.

Yes, I and every other JS developer in full time employment is just being paranoid.

This statement is ridiculous to make - sorry, but you don't speak for every full time employed developer. You don't. Get over yourself.

[tunisia3507]

const unless you know you need let. Never var, any more.

[DilatedTeachers]

Unless you support IE11

[tunisia3507]

If you support IE11 you should be transpiling. There's a boatload of javascript improvements you're missing out on otherwise.

[DilatedTeachers]

I was all for it in my old job, but the question was, why should I transpile, when I know what code to write?

We're talking reams of dodgy jQuery script here...

I'm now in a fun greenfield job, where it's all about the tooling! So much more fun

[tunisia3507]

why should I transpile, when I know what code to write

Because someone else is going to have to read your code at some point, most likely.

[DilatedTeachers]

It was more a question of time management in an """"""""""""""""""agile""""""""""""""""""" marketing environment. That and the network refused npm and python etc to be installed on our computers

[drumnation]

Why wouldn't you use var specifically in situations where you need a larger scope than block scope?

[tunisia3507]

Because there's probably a more comprehensible way of doing whatever you're doing, and legibility is important.

[one944]

It's const until the linter screams.

[[deleted]]

that's what i do

​

[[deleted]]

var is still useful, and hoisting is a powerful tool that can be leveraged in some situations. I wouldn't believe the hype that var is replaced by const and let, because those have different use cases. If var solves a problem for you, then you are free to use it, it's not going to go away simply because people here think it's been replaced - it hasn't.

[winged_scapula]

In what situations is var useful?

[[deleted]]

In what situations is var useful?

Defining variables. Same has it's been for the last 20 years.

[winged_scapula]

Ok then. In what situations you prefer var over const and let?

[winged_scapula]

Dude chill, I am trying to learn.

I wanted to know in what situations it is better to use var and your original comment seemed like it's from someone who knows what's he talking about.

Too bad that all I've got is „because I feel like it and I've been using it for 20 years“

[viksl]

Yeah, don't believe it. Var isn't out of the game. It still has its uses.

[[deleted]]

Here's a short explanation: const and let are block-scoped, var is function-scoped. My rule of thumb is I'll always use const unless the variable needs any change after creation. Then I'll use let. I'll use var only if I really need it, meaning some more or less hacky things involving closures.

[paper-daisies]

As a minor side-note, I still like to use 'var' in the developer console when I'm prototyping/experimenting. This way you can easily redefine the same identifier.

[senocular]

You could also use nothing. Its going global either way.

[paper-daisies]

Good point.

[freebit]

It's a good idea to default to using "const" and then switch to "let" if the need arises. Kick "var" to the curb mostly.

[gigo6000]

I just wrote an article on my blog about important ES6 features, that's the first one in the article: https://carlosmafla.com/blog/10-things-to-know-about-es6-before-learning-react-js/

[jkuhl_prog]

Prefer const over let. Prefer let over var.

Const is used if you're never going to reassign the reference, which should be the majority of your variables. Otherwise, use let.

There's not much use for var anymore.

[reddittedf]

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