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

whoadarshpandya profile image Adarsh Pandya ・3 min read

Before we start

πŸ‘‰ Before we start this article i would like to clarify some technical jargons for you

πŸ‘‰ Scope : Scope is nothing but a code block where the variable is accessible for usage

πŸ‘‰ 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

πŸ‘‰ 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...)

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

Var

πŸ‘‰ Var is the oldest way of declaring variable

πŸ‘‰ Var can be globally and functionally scoped

πŸ‘‰ 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

πŸ‘‰ can be redeclared or updated

πŸ‘‰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);
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πŸ‘‰ output

-> inside function
10 βœ”
20 βœ”
-> outside function
10 βœ”
uncaught reference : b is not defined ❌ 
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πŸ‘‰ Notice that functions can access both global and functional variables.

πŸ‘‰ Example of Re-declaration

// variables with var can be re-decalred βœ”
var a = 10;
var a = 20;
// you won't get any error
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πŸ‘‰ Example of updatable variables

// variables with var can be updated βœ”
var a =10;
a=20;
// you won't get any error
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πŸ‘‰ Problems with var

πŸ‘‰ 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

πŸ‘‰ Let is the modern way of declaring variables introduces in ES2015

πŸ‘‰ Let is now recommended way of declaring variables

πŸ‘‰ Let is block scoped

πŸ‘‰ Let can be updated but not redeclared

πŸ‘‰Example of Declaration

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

// error : Identifier 'b' has already been declared
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πŸ‘‰Example of block scope

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

console.log(sayHi); βœ”
console.log(newMsg); ❌ // 'newMsg is not defined'
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Const

πŸ‘‰ Variables declared with const remains same throughout the execution

πŸ‘‰ variables declared with const are not redeclarable or updatable

πŸ‘‰ if const variables declared outside any block they becomes global scoped , but if they are declared within block they becomes block scoped

πŸ‘‰ values can not be changes or new values can not be assigned to const variables

πŸ‘‰ 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.
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πŸ‘‰ But...

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

// but...
numbers = [5,6,7]; ❌ // won't work
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πŸ‘‰ 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. πŸ˜€


Summary

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


πŸ‘‰ let: block scoped. can be updated but we can't redeclare.


πŸ‘‰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' ConnectπŸ‘‹

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Discussion (10)

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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 πŸ˜‚

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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 :)

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matjones profile image
Mat Jones

Definitely agree. Just pointing it out.

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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

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matjones profile image
Mat Jones

just saw your comment after posting mine

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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 :)

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matjones profile image
Mat Jones

Definitely agree. Just pointing it out.

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theshakeabhi profile image
ABHISHEK CHANDRASENAN

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

var a = 10;
var a = 20;
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?
Or did you mean what you wrote:

var a = 10;
var b = 20;
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?

Do feel to correct me if I am wrong.

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whoadarshpandya profile image
Adarsh Pandya Author

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

Thanks

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theshakeabhi profile image
ABHISHEK CHANDRASENAN

Glad I could help.