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Paul Thompson
Paul Thompson

Posted on • Updated on

3 reasons to use 'var' in JavaScript

The var keyword was JavaScript's first way to declare a variable. It fits, right? Var, variable, it's in the name. But like the Joker, var is more than pretty face -- it has some baggage. As a beginning programmer, or a seasoned one dipping your toe into the seedy underbelly of JavaScript development, you maybe be wondering when you should use var to declare your JavaScript objects and functions.

Never fear. I have 3 reasons for you to use var in your next JavaScript project.

You love tradition and spurn anything new

Var may be the oldest keyword to declare a variable, but it is by far the only one. It's younger cousins, let and const exploded onto the scene with ES6 (over 6 years ago as of this writing!). But if sticking to old norms fraught with perils is your thing, keep using var and leave these upstart keywords to listen to their terrible pop music and complain about their parents.

You prefer mutability, even when you don't

Var has the troubling distinction of creating a variable of any type, which can then be changed to a variable of any type later. That's right, the number you declared on line 4 just became a string on line 17 and a boolean on line 109. This type flexibility is a key feature of JavaScript, but it can be avoided. But if you too like to live dangerously, keep using var.

You like leaky scopes

Prior to ES6, JavaScript variables were always defined at the function scope. In a lot of cases, this meant variables were global. JavaScript didn't enforce block-level scopes such as inside a for loop or if block. So a variable declared with var would be hoisted to the top of its function scope. In other words, the variable declaration moved to the top of the current lexical environment with a value of undefined. A common issue arising from hoisting is that such variables are accessible outside the block scope, where the developer may not expect them to be.

A subtler and more difficult to detect bug can happen here as well, variable shadowing. Rarely this may be desired. If that is you, var is your hero.

There must be a better way

If you made it this far, you're probably seconds away from pounding out a fierce comment about how I'm wrong about var. The truth is, no one should be using var in any JavaScript code except for a history lesson. Even then, it should come with a footnote that says something like, "please don't ever do this."

Let and const have been available in every major browser for the last 6 years. This is really, really ancient in frontend years. var isn't just the parent misunderstanding their teen, it's the great grandparent everyone loves but no one invites to the fun parties.

Let retains JavaScript's flexible type system while enabling block-scoping. Const creates a -- you guessed it -- constant variable in block-scope. Const creates a variable that cannot be reassigned, but similar to most other languages, const objects can have their properties mutated. These two alternatives to var should cover every use case you have. Even if you are in the tough spot of needing to support truly archaic browsers, don't use var. Use a tool such as Babel.

So please, please, do not use var.

Top comments (66)

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lukeshiru profile image
Luke Shiru

I fell in the click-bait trap! I was about to write a comment about why you shouldn't use var until I started reading 🀣 ... Indeed, nowadays there's no excuse to use var instead of let or const.

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paritho profile image
Paul Thompson Author

And yet I keep seeing it time and time again in tutorials. Sad.

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thumbone profile image
Bernd Wechner • Edited on

All language updates take time to trickle through. It is the way of the internet. I mean search the web with any technical question and rail, as most of us do, at any articles that are not presenting their publication date clearly, as stuff simply gets dated but still always found. I think we'll be reading Python 2 solutions to problems on web search results for years to come yet for example ....

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digitaldevourer profile image
DigitalDevourer

Hate when that happens.
You're Β½ way through when things start to feel off, and now it's become a hunt for the date.
Worst of all are the "SEO-based" articles that update their titles or keywords to match popular searches, despite having almost no related content or are years too late.

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baenencalin profile image
Calin Baenen

My school uses Code HS with this dog named Karel, and a pet peeve of mine is that they use var.
The teacher doesn't even address them, as they basically have no education on the language itself (they're just permitted and appointed to teach it).

Even as someone who barely uses JS, this annoys the fuck out of me.

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emptyother profile image
emptyother

6 years is nothing! I've seen PHP tutorials with methods 18 years past deprecation.

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lukeshiru profile image
Luke Shiru • Edited on

Let me fix that for you:

6 years is nothing! I've seen PHP tutorials! 🀣

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darkwiiplayer profile image
π’Š©Wii πŸ’–πŸ’›πŸ’šπŸ’™πŸ’œπŸ’πŸ’Ÿ

Same here, I also came here looking for outrage but then read the headings :D

Top tier clickbait

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getify profile image
Kyle Simpson
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lukeshiru profile image
Luke Shiru

One thing I don't quite get about the points being made there is that they complain about this:

let greeting = "Hi!";

{
    // what should print here?
    console.log(greeting);

    // .. a bunch of lines of code ..

    // now shadowing the `greeting` variable
    let greeting = "Hello, friends!";

    // ..
}
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Because we get an error, but my question is: In which scenario is desirable to have a shadowed variable that we assign after is used? That code above is actually preventing bugs:

  1. The dev maybe wanted to use the external greeting and got confused, the error will help with that.
  2. The dev maybe wanted to use the internal greeting value, the error will help with that too.

So, to fix 1 the dev could:

let greeting = "Hi!";

{
    // This prints the external `greeting`
    console.log(greeting);

    // No more shadow, the above code works, no errors.
    let longGreeting = "Hello, friends!";
}
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And to fix 2:

let greeting = "Hi!";

{
    // Moved shadow to the top
    let greeting = "Hello, friends!";

    // This prints the internal `greeting`
    console.log(greeting);
}
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In both scenarios, the greeting in the upper scope is not being changed, which is ideal because we are writing code that declares a new value, we shouldn’t be changing pre-existing code. If you really want to change the original value of greeting, you need to be more explicit about it (which is great):

let greeting = "Hi!";

{
    // This prints the value of `greeting` in the upper scope
    console.log(greeting);

    // Change value of upper scope `greeting`
    greeting = "Hello, friends!";
}

// Out here `greeting` has a new value now
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Is there any scenario in which that is bad from your point of view? I would personally take that behavior over var's any day. With var:

var greeting = "Hi!";

{
    // This will print "Hi!"
    console.log(greeting);

    // Maybe you want to do something with this, maybe you don't
    // we'll never know...
    var greeting = "Hello, friends!";
}

// Oops `greeting` out here now is "Hello, friends!"
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I agree that var was never broken, but Function.prototype isn't broken either, and folks prefer to use class. Nobody said var is broken, it just has a different way of dealing with scope and hoisting that is worst than "the new thing". You can change every use of let and const with var, but why would you? You could also use Function.prototype pretty much in every place you're using class but you wouldn't. I also agree that const has limited utility, but still it has at least some utility. Ideally it should make objects immutable, yes, but that doesn't mean that because it doesn't has that is suddenly worst than var. Going back to the examples above, if you declare the upper scope greeting as a const then you get an error when you try to change its value, so if the idea was to avoid changes in the upper scope string, const gave you something that you didn't had with var or let.

TL;DR: I agree var was never broken, but the utility we get from let and const make them a little more useful, and that little is enough to use them over var from my point of view.

Cheers!

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getify profile image
Kyle Simpson

I don't think I hardly ever would take advantage of variable hoisting in this way:

x = 2;

// later:
var x;
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I agree that's problematic. But I don't want the language to complain at me, I want a configurable linter to complain at me. That's my objection to these errors. The language doesn't need to be my nanny. I am perfectly capable of choosing what rules I want enforced or not. And if it's a configurable and extensible tool, I can extend it to understand more nuances to when I want to allow disallow things. Once you bake an error enforcement into the language, you force that opinion on everyone, forever.

Moreover, there's many other ways that hoisting (and var) ARE helpful. For example:

try {
   var x = whatever(42);
}
catch (err) {
   var x = "oops";
}

console.log(x);
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Here, the try..catch construct is an "accidental" scope... IOW, I never intended it to be a scope. If I use let inside it, it becomes a scope when I don't want it to be. If I use let outside the block statement, I have to separate my declarations from my initial assignments, which I rarely like to do.

Another example:

do {
   var x = whatever(42);
}
while (x < 100);
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I generally prefer the loop to be treated like its own scope, but unfortunately, the while condition is not inside that scope, so I need the x to be in an outer scope. But it doesn't make any sense semantically to declare it there, since it belongs to the loop.

And also "function hoisting" is useful, for example in cases like this:

fn = wrapWithSomeBehavior(fn);

// later:
function fn(whatever) {
  // ..
}
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There are a variety of other edge cases and uses. But in summary: there are perfectly valid uses of var (or hoisting in general) that are not just stylistic preference or semantic in nature, but also functionally more desired.

We should use var when it's helpful, use let when it's helpful, and (occasionally) use const if we really must.

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lukeshiru profile image
Luke Shiru • Edited on

I get where you're coming from, but I don't see myself "opting out" of those errors, so having just a linter for that doesn't feel quite right. Taking your snippets as examples:

try {
    var x = whatever(42);
} catch (err) {
    var x = "oops";
}
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I read the above as try to declare x with the output of whatever, and if it throws, then declare x with another value. I instantly think that you actually meant to write "Assign the output of whatever to x, or "oops" if it throws, and to express that we can just use let:

let x;

try {
    x = whatever(42);
} catch (err) {
    x = "oops";
}
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Same for the while, I read your code as "declare x with the output of whatever", when I actually think that you wanted to change the value of x in every loop like this:

let x;

do {
    x = whatever(42);
} while (x < 100);
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And about your point with function hoisting, I'm one of those folks that prefer to use things after those things are defined, so I would never write code like:

fn();

function fn() {}
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I generally write the function first, and then I use it, so it would look more like this:

const fn = () => {};

fn();
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And I generally use const because once I create declare a function, I don't have any intention of changing that declaration at runtime, so receiving an error if me or someone else tries to do fn = wrapWithSomeBehavior(fn) is valuable for me.

Thanks for taking the time to reply, Kyle! I follow your work since I read one your posts in David Walsh blog (I used to be a fan of MooTools back in the day, so I followed David's work closely), and I find extremely useful to have this kind of dialogs with folks like you!

Cheers!

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getify profile image
Kyle Simpson

Here's the thing: you're entitled to your opinions on these things and I'm entitled to mine. I strongly disagree with every point you just made. But that doesn't really matter. Neither one of us is "right". So why should the language codify one set of opinions and make life harder for those who don't agree? Tools are where opinions belong, not languages.

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lukeshiru profile image
Luke Shiru

Yup, I get it, that's why I mentioned that I find it valuable (and it seems there are lots of folks that do too). The good thing is that you still have var if you want to code with your style, and I can just avoid var to code with mine.

And to be completely honest, I don't like "throwing" in general, I prefer other ways to handle errors, so in that regard I also agree with you that at least using let/const "wrong" shouldn’t throw.

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lonyyy profile image
lonyyy

lmaoo your explanation is more complete than the actual article

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sabbakilam profile image
SabbaKilam

Thanks for the rational clarification. Nuanced treatment of this topic is rare.
I've never liked const. I'll start using var again for function scoping.

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sabbakilam profile image
SabbaKilam

Thanks for the rational clarification. Nuanced treatment of this topic is rare.
I've never liked const. I'll start using var again for function scoping.

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dagr8 profile image
DAGr8

Cancel VAR
The cancel culture is real. ;p

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thebearingedge profile image
Tim Davis

I teach var first because:

  1. It's still out there and I don't want my students seeing it at work and thinking "why didn't my teacher tell me about something so basic?"
  2. It behaves differently from let and const and it's completely different to have a pure academic or a "theoretical" understanding of something and having real hands-on (for more than an hour or some tweet's worth of words) experience with something.
  3. Knowing about and being competent when dealing with all three is better than knowing about and being competent when dealing with only two of the three (66% is a D, great job).
  4. Block scoping is a good transform output to demonstrate when teaching Babel.
  5. I ask for let and const to be used later in my curriculum where appropriate.

I don't really know who this article is for, but sadly it seems to encourage ignorance in exchange for cool coins.

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lukeshiru profile image
Luke Shiru

Did you teach them about eval, with and other old stuff as well? I mean I get that you want to teach old stuff for context, but you should be very clear about avoiding var. Babel no longer does those transformation from let and const to var unless you explicitly configure it to do so. Go and check the Babel REPL.
I agree we should know about var, but one thing is knowing about it and other is actually using it.

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thebearingedge profile image
Tim Davis • Edited on

Did you teach them about eval, with and other old stuff as well?

that's not really what this is about.

I mean I get that you want to teach old stuff for context, but you should be very clear about avoiding var.

I've yet to hear any real argument for this.

Babel no longer does those transformation from let and const to var unless you explicitly configure it to do so. Go and check the Babel REPL.

I addressed this already.

I agree we should know about var, but one thing is knowing about it and other is actually using it.

My point is that you don't know something unless you have used it.

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lukeshiru profile image
Luke Shiru

I mean I get that you want to teach old stuff for context, but you should be very clear about avoiding var

What I meant by that is that I get that you teach that to give them context about how we used to do stuff with var, including examples or even exercises with it. But My point is that the same way you could teach eval, you need to be super clear about the limitations and problems it has. Maybe even center the exercises you give to your students in this problems so they struggle with them and when you present let and const, they see why are they appealing to devs nowadays.

About your answer:

I've yet to hear any real argument for this.

Several where made in this article already, but it boils down to your coding style. For mine, let and const are way more useful than var, so I avoid var and in my personal projects I have linting rules against it. There are folks with other coding styles that rely on hoisting, redeclaration and leaky block scopes that benefit from var, so for them it makes sense to do the opposite and stick to var and maybe even ban let or const in their linting rules.

One thing is clear, and is that is way more popular to prefer let/const, but the good thing is that if you don't, JS is designed to "not break old code", so you can just stick to var forever if that's what you prefer.

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thebearingedge profile image
Tim Davis • Edited on

For mine, let and const are way more useful than var, so I avoid var and in my personal projects I have linting rules against it.

Same here. But I teach beginners and coding style is the least of their worries and they need to be prepared to slot into the team they're hired onto.

There are folks with other coding styles that rely on hoisting, redeclaration and leaky block scopes that benefit from var, so for them it makes sense to do the opposite and stick to var and maybe even ban let or const in their linting rules.

This is an interesting observation that I've seen as well. IME, they just write sloppy or confusing code in general. It's always possible to write code that avoids the pitfalls of var entirely as long as the style is reasonable. What's weird is that the style of my reference solutions function identically when you refactor them to the "right" way and use const / let. This is probably why it's so rare for my students get "tripped up" by the pitfalls of var. When they do, the code they're writing makes no sense anyway, which is expected of beginners.

One thing is clear, and is that is way more popular to prefer let/const.

Yes, you are absolutely right, which is why they're also equipped for that. The motivation behind my comments also boils down to your point. There are social pressures and media being produced that discourage thinking critically about how things work, and when they're appropriate. Often things get reduced to memes and jokes and if I "warn" them about anything, it's that they're going to encounter people, videos, and writings that are reductive and unproductive. They need to be careful not to trick themselves into thinking they're any better at building useful software by "agreeing" with anybody's lopsided opinions.

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getify profile image
Kyle Simpson

I taught eval and with for years, specifically so people understood why they shouldn't be used.

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raibtoffoletto profile image
RaΓ­ B. Toffoletto

Ufff.... I came here hoping that the title was sarcastic, but for a moment in the first paragraph I worried that you were REALLY to defend VAR.... 🀣. Nice writing πŸ‘ and let's all chant "die VAR, die...."

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koire profile image
Koire

Var is actually used a lot in enterprise

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lukeshiru profile image
Luke Shiru

Don't be that guy. We all know that "enterprise" have lots of deprecated stuff, but still that doesn't justify using it. Just because there are some banks still using COBOL, that doesn't mean you should learn it.

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koire profile image
Koire • Edited on

We don't always write var but for IE but we definitely transpile it
I still have projects that warn you you need IE 5 or later for it to work correctly.

Edit
There is the case where someone might come in and say "var bad" and use replace to turn them all to let or const and break everything in a legacy system because it was being funky

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raibtoffoletto profile image
RaΓ­ B. Toffoletto

I'm not saying to go back and alter legacy code, but in 2021 you should not be writing JS with var... enterprise is a ship difficult to steer, but you can do it slowly. I don't shame on my colleagues still using jQuery to build stuff, but you can use const and let with jQuery.

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emptyother profile image
emptyother

Two "good" reasons I still use var:

  • ~Old projects that need to support IE11 and isnt transpiled.~ Oh, right, even IE11 supports let and const. Well almost, it still have a few gotchas that will never be fixed.
  • I'm also writing C# and var is stuck in my fingers. Sometimes I even miss the eslint warnings about it. Darn muscle memory.
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aequinn profile image
Anthony Quinn

Number 3: kind of like old projects, you are doing any JavaScript variables in Google Tag manager, they still force you to use var, I develop the scripts with the new and am forced to roll it back to the old when try to save.

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mtrantalainen profile image
Mikko Rantalainen • Edited on

The only reason to use 'var' is to be compatible with some old info tv systems which have some weird more or less broken HTML renderers. There you cannot use any automated tools either because nobody really wants to support those things so you have to write fully custom implementations. If you don't need to support such things you shouldn't use 'var' anymore.

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zalithka profile image
Andre Greeff

disclaimer: let me start by saying that this is a very highly contested topic in JS.. with many different people throwing many different reasons for different using any one of these three over the others...

I would put myself quite firmly on the "var is dead! long live var!" bandwagon here... and not just because it leaks out of the current block scope. :p

however, to actually consider the full value of var, you need to keep in mind that it came about long before "block scoping". even just variable hoisting itself was not just one of those weird situations you needed to be aware of, it could be explicitly (ab)used to do some interesting things.. even if these were not considered "good practice", either way-back-when or in the current day.

the most annoying part of many blog posts on this topic, has to be the nature of the coding examples given: where something will be shown leveraging the default behaviour of let (with it's block scoping restriction), then followed up with an overly verbose and borderline hacky implementation of the same final result using var... and to top it off, this is all this before we even start on the accompanying changes introduced in ES6, since that is the real core of this eternal var vs let/const debate..

--

topics like this feel akin to comparing a water bottle with a water tanker; they both hold liquid, they can both be opened and closed, they can both be moved, etc. etc., but they do so at different levels, in different ways, for different reasons.. so can we really compare them directly?

for variable assingment, we now have three different ways to assign something to a variable name. use the one that fits your needs, change it if you need to, and carry on with life. that's all there is to it.

at the end of the day, I would not recommend that anybody entirely abandons var, or even avoids let/const... my suggestion is to work at understanding the quirks and gotchas of each, and to use whatever is most appropriate for your current requirements.

@paritho I would suggest you amend your post a little, if for no other reason that to change the last sentence to read:

So please, please, do not use var without carefully considering your current unique use case.


this did make me chuckle, quite loudly if I'm being honest:

var isn't just the parent misunderstanding their teen, it's the great grandparent everyone loves but no one invites to the fun parties.

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darkwiiplayer profile image
π’Š©Wii πŸ’–πŸ’›πŸ’šπŸ’™πŸ’œπŸ’πŸ’Ÿ

A friend was learning JS at school until not long ago, and they were still learning var, HXR and function (as in, unscoped function declarations that end up in window by default).

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sabbakilam profile image
SabbaKilam

I always use var at the browser's console so that I can re-declare and re-assign variables at will, hassle-free, while playing around and testing concepts. Otherwise, I get clobbered with error messages and restrictions.

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lukeshiru profile image
Luke Shiru

In recent versions of Chrome, you can "redeclare" const and let in the console πŸŽ‰

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andyhilton profile image
Andy Hilton

Why didn't let just replace var? If the whole world is against it why does it still exist?

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paritho profile image
Paul Thompson Author

The EcmaScript committee has strict guidelines to 'not break the web'. Meaning, old JS code must always work.

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koire profile image
Koire

Also let is scoped

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emptyother profile image
emptyother

Nobody want to dig up and rewrite their 10 year old production code. But eventually it too will go the way of elem.attachEvent() and the <blink> tag.

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andrebradshaw profile image
Andre Bradshaw

Let and const are useful, but when people say var should never be used, I immediately start to question if they have done any interesting development in JS.

Additionally, for beginners, I would argue that using var forces people to deal with unexpected behaviors which is a useful method for actually learning how JS works.

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matttepp profile image
MatyΓ‘Ε‘ TeplΓ½

They had us in the first half, not gonna lie.

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meliodas profile image
Henry Onyango

There are reasons to still use var. There's nothing with var. It's not broken. It's important you understand how it works and when is most appropriate to use it.

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paritho profile image
Paul Thompson Author

I'd be interested in hearing when it is appropriate to use it

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meliodas profile image
Henry Onyango

Look at the section "making the case for var"
github.com/getify/You-Dont-Know-JS...

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lukeshiru profile image
Luke Shiru

The author of the book is in the comment section as well. His point is mainly that if you have a certain coding style, var is better than let and const, and if you have other style, let and const are better. What we agree on is that it shouldn’t throw.

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mustapha profile image
Mustapha Aouas

I strongly disagree with every point you just made

I don’t think he agrees on anything with you.

Cheers!

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lukeshiru profile image
Luke Shiru

Gotta love quotes without context.

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koire profile image
Koire

Don't forget code golf!
Let's scope is more constrained than var, so you (maybe) can't do as many tricks with let as var.
And const is 2 more characters! That's unforgivable

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paritho profile image
Paul Thompson Author

There are actually a lot of really cool exploits you can do in JS because of it. Golf coding is amazing!

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yamanidev profile image
Mohamed El Amine Yamani

As soon as I read the title, I didn't give a damn if it was a clickbait or not, I am reading this anyway xD. I like the humor, keep it up!

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This post blew up on DEV in 2020:

js visualized

πŸš€βš™οΈ JavaScript Visualized: the JavaScript Engine

As JavaScript devs, we usually don't have to deal with compilers ourselves. However, it's definitely good to know the basics of the JavaScript engine and see how it handles our human-friendly JS code, and turns it into something machines understand! πŸ₯³

Happy coding!