Var vs Let vs Const in JavaScript

#javascript #reactnative #react #oop

Let's understand comparison between var, let and const.

Variables defined with var declarations are globally scoped or function scoped and let and const has block scope or we can say local scope.

example:

var testVar = 'var'; // global scope
let testLet = 'let'; // local scope
const testConst= 'const'; // local scope

function testScope {
  consol.log(window.testVar); // var
  consol.log(window.testLet); // undefined
  consol.log(window.testConst); // undefined
}

testScope() // output var

Variables defined with var can be Redeclared but with let and const cannot be Redeclared.

example :

var testVar = 10; 
let testLet = 10;
const testConst = 10;

function test{
   var testVar = 20; // it will work
   let testLet = 20; // it will throw error
   const testConst = 20; // it will throw error

console.log(testVar,testLet,testConst); 
}

test();

Var can updated and re-declared.

Let can be updated but not re-declared.

Const cannot be updated and re-declared.

example :

var testVar = 10; 
let testLet = 10;
const testConst = 10;

function test{
   testVar = 20; // it will work
   testLet = 20; // it will work
   testConst = 20; // it will throw error

console.log(testVar,testLet,testConst); 
}

test();