DEV Community

loading...
Cover image for Enough JavaScript to get you Started : #16 var vs let vs const

Enough JavaScript to get you Started : #16 var vs let vs const

whoadarshpandya profile image Adarsh Pandya ・3 min read

Before we start

👉 Before we start this article i would like to clarify some technical jargons for you

👉 Scope : Scope is nothing but a code block where the variable is accessible for usage

👉 Global Scope : Global Scope means variable is declared globally (not in some condition or function) hence it can be used any where throughout the execution of program

👉 Local/ Functional Scope : this simply means that when we declare a variable on function level or somewhere locally in code block then it's not accessible outside of that particular scope (imagine variables declared in functions , loops ,conditionals...)

👉 Block Scope : Blocks are nothing but piece of code written inside any curly braces {...} [ex. if block , or function's block]

Var

👉 Var is the oldest way of declaring variable

👉 Var can be globally and functionally scoped

👉 If we Declare var inside function it becomes functionally scoped if we declare it outside function it becomes globally scoped and is available anywhere in program

👉 can be redeclared or updated

👉Example of Scope

var a = 10; // global variable

function fun()
{
    // functional scoped variable
    var b = 20;
    console.log(a);
    console.log(b);
}
fun();
console.log(a);
console.log(b);
Enter fullscreen mode Exit fullscreen mode

👉 output

-> inside function
10 ✔
20 ✔
-> outside function
10 ✔
uncaught reference : b is not defined ❌ 
Enter fullscreen mode Exit fullscreen mode

👉 Notice that functions can access both global and functional variables.

👉 Example of Re-declaration

// variables with var can be re-decalred ✔
var a = 10;
var a = 20;
// you won't get any error
Enter fullscreen mode Exit fullscreen mode

👉 Example of updatable variables

// variables with var can be updated ✔
var a =10;
a=20;
// you won't get any error
Enter fullscreen mode Exit fullscreen mode

👉 Problems with var

👉 Redefining variables won't throw any errors which means it's tricky to remember which variable is already there and which variable is new.


Let

👉 Let is the modern way of declaring variables introduces in ES2015

👉 Let is now recommended way of declaring variables

👉 Let is block scoped

👉 Let can be updated but not redeclared

👉Example of Declaration

// let can be updated ✔
let a = 30; ✔
a = 40; ✔
// but not redeclared ❌
let b = 40; ✔
let b = 90 ;❌

// error : Identifier 'b' has already been declared
Enter fullscreen mode Exit fullscreen mode

👉Example of block scope

let sayHi = "hi";
if(sayHi === "hi")
{
    let newMsg = "how are you?";
   console.log(sayHi); // outputs "hi"
}

console.log(sayHi); ✔
console.log(newMsg); ❌ // 'newMsg is not defined'
Enter fullscreen mode Exit fullscreen mode

Const

👉 Variables declared with const remains same throughout the execution

👉 variables declared with const are not redeclarable or updatable

👉 if const variables declared outside any block they becomes global scoped , but if they are declared within block they becomes block scoped

👉 values can not be changes or new values can not be assigned to const variables

👉 Example:

const sayHi = "hi";
const sayHi = "hello"; ❌ // will throw error

//and ...

const msg = "buy bread from market";
msg = "new msg here"; ❌ // error:  assignment to constant variable.
Enter fullscreen mode Exit fullscreen mode

👉 But...

const numbers = [1,2,3];
number[0] = 4; ✔ // works fine

// but...
numbers = [5,6,7]; ❌ // won't work
Enter fullscreen mode Exit fullscreen mode

👉 the first case will work because we're within the rules , we're not redeclaring the const variable nor we're updating it. but we're mutating it. 😀


Summary

👉 var: Global/function scoped depending on declaration undefined when accessing a variable before it's declared. can be redeclared and updated.


👉 let: block scoped. can be updated but we can't redeclare.


👉const: block scoped. can not be reassigned nor we can redeclare.

Let me know in comment section if you have any doubt or feedback. it's always worth to give time to the thriving developer community :)

Keep Coding ❤

Hey , Let' Connect👋

Twitter / Github

Discussion (10)

pic
Editor guide
Collapse
matjones profile image
Mat Jones

I've been doing software professionally for several years now and I honestly cannot think of a single real-world case where I've ever had to use var or any case where var could possibly have been better in any way. Basically vars only use is for const and let to be compiled to it for Internet Explorer support 😂

Collapse
whoadarshpandya profile image
Adarsh Pandya Author

I totally agree, I don't use var , but this is for beginners so it's beneficial to compare :)

Collapse
matjones profile image
Mat Jones

Definitely agree. Just pointing it out.

Collapse
aidap1 profile image
Aidan

Is there any reason to use var?
Everything I've been taught (only in the last ~1 month, I'm new) has focused on let and const

Collapse
matjones profile image
Mat Jones

just saw your comment after posting mine

Collapse
whoadarshpandya profile image
Adarsh Pandya Author

I totally agree, I don't use var , but this is for beginners so it's beneficial to compare :)

Thread Thread
matjones profile image
Mat Jones

Definitely agree. Just pointing it out.

Collapse
theshakeabhi profile image
ABHISHEK CHANDRASENAN

Hey,
In the Var section of "Example of Re-declaration",
Did you mean:

var a = 10;
var a = 20;
Enter fullscreen mode Exit fullscreen mode

?
Or did you mean what you wrote:

var a = 10;
var b = 20;
Enter fullscreen mode Exit fullscreen mode

?

Do feel to correct me if I am wrong.

Collapse
whoadarshpandya profile image
Adarsh Pandya Author

Hey you're right, it was typo - fixed it

Thanks

Collapse
theshakeabhi profile image
ABHISHEK CHANDRASENAN

Glad I could help.