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Manthan Ank
Manthan Ank

Posted on • Updated on

Understanding the Differences Between JavaScript's var, let, and const

var let const
global scoped yes no no
function scoped yes yes yes
block scoped no yes yes
can reassignable yes yes no
can be redeclarable yes no no
can be hoisted yes no no

Global Scoped :

Variables declaredΒ GloballyΒ (outside any function).

//var
a = 4;
var a;
console.log(a);

//let
a = 4;
let a;
console.log(a); // Uncaught ReferenceError: Cannot access 'a' before initialization

//const
a = 4;
const a;
console.log(a); // Uncaught SyntaxError: Missing initializer in const declaration
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Function Scoped :

Variables defined inside a function are not accessible (visible) from outside the function.

//var
function myFunction() {
    var a = 10;
}
myFunction()
console.log(a);

//let
function myFunction() {
    let a = 10;
}
myFunction()
console.log(a);

//const
function myFunction() {
    const a = 10;
}
myFunction()
console.log(a);
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Block Scoped :

This scope restricts the variable that is declared inside a specific block, from access by the outside of the block.

//var
function myFunction () {
    if(true) {
        var a = 10; // it exists in function scope
    }
    console.log(a);
}
myFunction();

//let
function myFunction () {
    if(true) {
        let a = 10; // it exists in block scope
    }
    console.log(a);
}
myFunction();

//const
function myFunction () {
    if(true) {
        const a = 10; // it exists in block scope
    }
    console.log(a);
}
myFunction();
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Can be reassignable :

//var
var a = 1;
a = 30;
console.log(a); // 30

//let
let b = 1;
b = 30;
console.log(b); // 30

//const
const c = 1;
c = 30;
console.log(c); // Uncaught TypeError: Assignment to constant variable.

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Can be redeclarable :

//var
var a = 10;
console.log(a); // 10
var a = 12;
console.log(a); // 12

//let
let b = 10;
console.log(b); // 10
let b = 12;
console.log(b); // Uncaught SyntaxError: Identifier 'b' has already been declared

//const
const c = 10;
console.log(c); // 10
const c = 12;
console.log(c); // Uncaught SyntaxError: Identifier 'c' has already been declared
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Can be hoisted :

//var
console.log(a);
var a = 10; // undefined

//let
console.log(b);
let b = 10; // Uncaught ReferenceError: b is not defined

//const
console.log(c);
const c = 10; // Uncaught ReferenceError: c is not defined
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