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

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);

👉 output

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

👉 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

👉 Example of updatable variables

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

👉 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

👉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'

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.

👉 But...

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

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

👉 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 :)

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