DEV Community

Var, let and const- what's the difference?

Sarah Chima on October 25, 2017

A lot of shiny new features came with ES2015 (ES6) and since it's 2017, it's assumed that a lot of JavaScript developers have become familiar with ...

Read full post

abhijeetckar92 • Aug 14 '18 • Edited

this is a good article..however you should update your last example
const flow = {
message : "say Hi",
times : 4
}

const greeting = {
message : "say Hi",
times : 4
}

don't know how anybody who's reading this article missed that!

Abdu Shaker • Jul 20 '19

I noticed something did not add up but I didn't pay attention to the details since I got what I was looking for. Good catch, tho.

Bkmillanzi • Jan 7 '20

left me in blanks... I am glad you had pointed it out! Thanks'
and btw, the article is indeed one of the best!

Sarah Chima • Jan 8 '20

thank you

Ansh • Feb 5 '20

It's updated now. Thanks for notifying

rama-chowdary • May 22 '19

Yeah

Dominic Sears • Oct 25 '17

For a noob like me, this explained "let" declarations for me.

Sarah Chima • Oct 25 '17

I'm glad it did.

Jens Genberg • Oct 25 '17

Good post. I wasn't sure about these. Just knew that 'var' should be avoided.

I think you have typo here. Remove the second 'let' from the first code block. :)
So while this will work,
let greeting = "say Hi";
let greeting = "say Hello instead";
this will return an error.
let greeting = "say Hi";
let greeting = "say Hello instead";//error: Identifier 'greeting' has already been declared

Sarah Chima • Oct 25 '17

Thanks a lot for pointing that out. I'll correct it right away.

itayko • Jan 4 '20

Thank for a great article

Error still exists in the article..

Ashley Brandon • Jul 31 '18

Why should var be avoided?

Scott Mallon • Dec 17 '18 • Edited

'Var' allows for re-declarations in code which does not throw errors and can create unintended side-effects in your code.

'Let' allows for variable reassignment but not for duplicate declarations (block-scoped), much like strongly typed languages like C# and Java.

'Const' allows for declaration once and for assignment once, and can never be re-declared or reassigned (block-scoped). For instance, I use 'const' for inline function declarations, so that I don't accidentally redefine this function's behavior at some later point in time by mistake. Even though I am unlikely to do so, it is just safer for myself and others to work on.

To paraphrase Sarah more generically:

"While it is not a problem if you knowingly want a var to be a certain value at a certain point in code, it becomes a problem when you do not realize that this same var has already been declared/defined before or, even worse, has had other functions operate on this var without your knowledge."

So, in other words, it's a much better idea to use 'let' and 'const' in order have safer control over your variables and constants.

Oiza • Jun 7 '19

This was incredibly helpful, and a great TLDR!

Joel Olawanle • May 13 '20 • Edited

const alone does not guarantee protection for your data or let me say it does not protect your data from mutation,

Example:

const GREETING = {
    name : "Joel",
    info : "Goodday!"
 }

Though the above code makes use of const i can still update the values via the code below

GREETING.name = "Elijah";

But this could easily be avoided by making use of Object.freeze() to freeze our const variables.
Just add this line of code and you will discover that the values cannot be updated.

Object.freeze(GREETING); 

GREETING.name = "Elijah"; // This will now be ignored due to mutation

For more clarification check: freecodecamp.org/learn/javascript-...

Ariel • Jan 20 '19

Hi, great article but I think you have a mistake about the hoisting,you wrote:
"They are all hoisted to the top of their scope but while var varvariables are initialized with undefined, let and const variables are not initialized."
If you will look at the JavaScript Hoisting section in w3schools.com website,
you will notice that:"Variables and constants declared with let or const are not hoisted!"
Please check in:
w3schools.com/js/js_hoisting.asp

Siddharath shankar bhardwaj • Jan 25 '20

That's exactly I m thinking the new let and const in javascript are not hoisted..she wrote wrong things..u should correct this article..do not mislead people with this ??

Mohammed • Mar 5 '20

According to MDN and ECMA 2015 her words are right
check this developer.mozilla.org/en-US/docs/W...

Mark Duff • May 9 '20

Years later, this is still the most concise explanation and welcomed even by those of us who are not lightweights - but don't stray into the JS badlands much. You have great teacher chromosomes, Sarah. Ignore the nit-pickers - this is still the best explanation primarily because of its brevity.

Sarah Chima • May 10 '20

Thanks for the kind words, I really appreciate it.

Wolf00Bomber • Apr 18 '19

The article is well written, but the problem is with the visualization of the concepts, if only these were depicted in pictorial or tabular format, can have immense effect on readers. Thanks for the article thought.

Ayush Jain • Mar 5 '19

That's a great article. Inspired me to write:

Var is function-scoped and not block-scoped which means that constructs like if statements, while loops etc do not create new scopes while using var. It also means that, as var is function-scoped, if we do not create a function, we do not create new scope while using var.

Read More here:

Var vs Let in JavaScript

gamebecks • May 20 '19

Hi, you have said that variable declared as var will have global scope if declared outside a function. But the same can even be said for variables declared as let and const. They indeed are locally scoped but when done outside a function, they will have global scope like var.
Please correct me if am wrong?

NickSu86 • Jun 10 '20

Thank you for your post, I am a newbie to Nodejs and quite confusing on when to use let, const or var. As I saw some require a module with var while some with const, could you advise which is better when requiring a module? After reading your post, I think const is better to require another module because it will never change.
One more question could you please do me a favor to explain the memory usage between these three, is there any different other than scope, thank you

Erkan Okman • Aug 4 '19

var erkan = 3;
if (erkan == 3) {
var erkan = 2;
console.log(erkan); //2
}
console.log(erkan); //2

//////////////

let erkan = 3;
if (erkan == 3) {
let erkan = 2;
console.log(erkan); //2
}
console.log(erkan); //3

//////////////

const erkan = 3;
if (erkan == 3) {
const erkan = 2;
console.log(erkan); //2
}
console.log(erkan); //3

Victor Sanchez • Apr 9 '20

I've finally understood.
(Again) 'Cause I had forgotten hehe 😹😹
Thanks ♥

Ryan • Apr 4 '19

Thanks for the explanations on the differences between var, let and const. When I first heard of let and const I was so confused. I wanted to know though is var still good to use and how should it be used when compared with let and const?

hexiaosheng • Apr 29 '19

very helpfully, many thanks

Nagendra • Sep 8 '19

Thank you so much. It was very easy to understand.

Sameer Gurung • Apr 21 '20

A very good explanation indeed. I was impressed as you did not miss updating of const object properties since most of us might think that anything declared as const cannot be updated :).

Lance Johnson • Apr 24 '20

Thanks Sarah! Great, simple explanation.

Scott Mallon • Dec 17 '18

The cleanest explanation I've seen so far of let vs. const vs. var.

Mahesh Nandam • Mar 13 '19 • Edited

Demystified the topic with your narration, Thank you.

Msahith • May 6 '20

absolutely nice article ,thumbs up

Kelvin Graddick • May 6 '20

This was great; I learn a lot and tightened up my knowledge in 5 minutes 🙌🏾

Okpala Chuks • Oct 26 '17

Nice piece... ES6 all the way!

Sarah Chima • Oct 26 '17

Thanks Chuks :)

Otome Charles Opuoro • Feb 13 '20

Thanks a whole lot Sarah.

Thoroughly enjoyed the article.

It was easy to understand.

Kind regards.

Pablo Martinez • Mar 24 '19

The best explanation that I see on Internet. Thanks!

Sarah Chima • Mar 25 '19

Thank you for your kind words.

Sarada Sutar • May 20 '20

Thanks, Good article on concept of using var, let and const.

Mengnan Shi • Nov 28 '18

Well explained!

chetanupreti • Mar 27 '19

let me clear about const.

const used when we don't want to reinitialize the variable values.

const a=[2,3,4];

a[1]++; //doesn't give error;

a=[3,4]; //give error;

that's about const.

Paul • Aug 5 '18

Excellent article! Thanks!

Nikhil Chugh • Apr 11 '19

Thanks for this article, it really helps me understand these basic concepts well.
Thanks a lot!

Rahul Chandra • Jan 20 '20

Good Post.Thanks.

rahul bhatija • Oct 18 '18

Thank you Sarah! It helped :)

Sabuhi • May 12 '19

Just started Javascript and learned a lot from this article. Thanks a lot, great explanation!

nitish pandey • Oct 30 '17

Some typos in there though. You have used an object flow accidentally. Two varying illegal examples of const look identical to me. Do check. Or did I miss the point. ☺. Cheers

Sarah Chima • Oct 30 '17

Thanks

Abraham ghaemi • Jan 15 '20

nice :)

vinayp59 • Jul 28 '18

If i declare an array as const variable , Can i update the array?

Sarah Chima • Jul 28 '18

While you cannot reassign the array, JavaScript allows you to update it. Personally, I think that defeats the idea of using const. If you want to declare an array that will be updated, it might be more ideal to use let.

Nicola Solazzo • Nov 7 '19

Good article, if you are interested you could read my article on the same topic. Maybe an exchange of ideas :)
medium.com/notonlycss/difference-b...

Megan Richards • Nov 21 '19

Great article! Super thorough, but clear enough for a beginner like me to understand. Thank you!

Sarah Chima • Nov 21 '19

Thanks Megan, I'm glad you like it.

Ehsan • Jun 9 '19

Thank you.

mang0g0rilla • Jul 9 '19

Excelent article, thank yoU!

Abdu Shaker • Jul 20 '19

I love the simplicity and consistency of the examples. They made the difference very clear. Thanks a lot.

Akash Chandra • Mar 19 '18

Very well written.. Thanks.. It made this clear

Arun Kumar G • Aug 1 '20

In a global scope var will be associated with global object
where as let and const are not
ex: window.var_variable
not supported by let and const

Rakesh Mandal 🗯️ • Dec 31 '19

A good read Sarah! Thanks for letting me understand the big controversy between these 3 currently we have in our community 😁

Sarah Chima • Dec 31 '19

Thanks for reading

delubio de paula • Oct 4 '18

I'm glad I found you. No flowery language. Spot on. Thank you Sarah!!!

ehimze007 • Mar 6 '20

This piece makes sense a 101%. Thank you.

Joe Brennan • Mar 27 '20

Great explanation, very easy to understand

Joshua Biggs • Jan 10 '19

Thank you for the article! I didn't know about variable hoisting or why using let and const was preferred over var. Excellent to know!

Puneet Pandey • Dec 28 '18

Nice to know about var, let and const. Great article!

Shareni Azcárraga • Jan 31 '19

Thank you so much, really cleared things up for me!

Dastin Prater • Apr 5 '20

Awesome article. Very well structured and easy to understand.

Stanley Melgar • Jul 14 '21

Very useful~

Hnin N_M • Feb 22 '18

Great Article :) . Thanks so much .

ArunEdathadan • Jun 4 '18

Thanks, but how to use global let variable inside function

Sarah Chima • Jun 4 '18

You cannot declare a global variable in a function with let.

IndrakumarRC • Jul 19 '18

Good one, thanks so much!

techstack • Aug 15 '18

Var, Let and Const in Javascript
youtu.be/R45FhbZl5SE

Sajad Zarsanj • Nov 8 '18

Thanks a lot Sarah, Very helpful.

Lars • Dec 2 '18

This is great, THANKS!

ErnestPH • Mar 20 '19

Well explained. thanks!

Daddy-chi • May 19 '19

Very well done! Neat article. Thanks!

rama-chowdary • May 22 '19 • Edited

Can you please explain me the 3rd point in hoisting of
const

herichirimwami • May 30 '19 • Edited

what is a best way to make a declaration between var, let and const

Light • Sep 24 '19

i now understand the use of let and const. thank you :)

Pratik Agrawal • Oct 3 '19

perfect explanation for var, let and const!!

Tosa Silviu • Jan 3 '20

Great article for "var - let - const" explanation. I am just learning Javascript and this guide clears out why I was seeing on other people code, for using the "let" and "const" instead of "var".