JS var,let and const

var, let and const will be discussed with their scope, use and hoisting.

var keyword

• Variable declaration are processed before the execution of the code, means variable hoisting
• The scope of JavaScript variable declared outside the function is global

let keyword

• Let allows us to create a variable with the scope limited to the block on which it is used.

const keyword

• const is same as var, but can not re-assign the value nor re-declared
• In case of object we can re-assign the value because the object is immutable

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var keyword

Scope:var variable are globally scoped or function scoped

Example 1- global scope

var test=10;
function testJS()
{
console.log(test);
}
testJS();//10

Example 2 - function scope

function testJS()
{
var test=10;

}
testJS();
console.log(test);//error: test is not defined

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var variables can be re-declared and updated too.

var test =10;
var test =15;
OR
var test =10;
test =15;

Hoisting of var

In js a variable(var) can be used before it has been declared.

Example 3

test=5;
console.log(test);//5
var test;

Example 4

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

Variables and constants declared with let or const are not hoisted

problem with var Variables , it's override block scope by function scope

var test=10;
if(test>5)
{
var test=5;
}
console.log(test);//5

---

let keyword

Scope:let is block scoped {}.

let test=10;
if(test>5)
{
let test=5;
}
console.log(test);//10

let test=10;
if(test>5)
{
test=5;
}
console.log(test);//5

let variables can be updated but not re-declared

let test=10;
test=5;

let test=10;
let test=5;//error: Identifier 'test' has already been declared

---

const keyword

const declarations are block scoped{}, it' same as let but const variable cannot be updated or re-declared.

Example 5 - Scope

const test=10;
if(test>5)
{
const test=5;
console.log(test);//5
}
console.log(test);//10

Redeclare

Example 6

const test=10;
test=5;//Uncaught TypeError: Assignment to constant variable.

Example 7

const test=10;
const test=5;//Uncaught SyntaxError: Identifier 'test' has already been declared

Example 8 - (Re-assign in case of Object)

const test={'name':'JS'};
test.name='JS6';
console.log(test.name);//JS6

Const Hoisting

test=5;
console.log(test);
let test;
//Uncaught ReferenceError: Cannot access 'test' before initialization

var vs let vs const

• var declarations are globally or function scoped,let and const are block scoped{}.
• var variables can be updated and re-declared within its scope, let variables can be updated but not re-declared,const variables neither updated nor re-declared.
• var and let can be declared without initializing it, const must be initialized during declaration.
• Only var variables are hoisted on the top,let and const variables are not initialized