Understanding Let , var , const in Javascript

Welcome to day 1 of the Interview Challenge in Javascript.

The interviewer might ask questions like :

A) What are variables in javascript?

B) Explain the difference btw let, var, and const.

C) Why var is not recommended in Javascript ES6 ?

D) What are block and function scope ?

E) Explain hoisting of variable and difference in var and let on the basis of hoisting ?

Variable in Javascipt :

Variable in Javascript are containers to store the data values with in our programs .

In Javascript , there are three types of variable declaration . Let ,var and const . LET used for block -scoped variables , meaning they're only accessible within the block ( under the curly braces {}) , Const is similar to let in scope but the value cannot be reassinged once it is intialized . While Var used for global scope means it can be redeclared in the same scope with same name .

Difference btw Let , var and const :

LET: Let variable used for block scope variable , they are not hoisted to the top of their scope it can be reassigned the value after intialization.

let name = 'alok tamrarkar ';
let surname = name;
let name = 'ankita';
console.log(name) // ans = ankita
console.log(surname); // ans =  alok tamrakar 
function name(){
let name = 'alok';
console.log(name); // alok 
}
name();

Const : Const is similar to let on scope . they are not hoisted to the top of their scope it cannot be reassinged the value once it is intialized.

const name = 'alok tamrkar ';
name ='anmol'; // not possible 
console.log(name) // ans = alok tamrkar

Var : Var used for global scope variable . they are hoisted to the top of their scope , it can be redeclared in the same scope with the same name .

var name = 'aok ';
console.log(name); // aok 

function naming(){
var name = 'ankita';
console.log(name);// ankita
} 
naming();

let surname = 'alok';

function surnaming (){
console.log(surname); // undiefined because let uses block scope 
}
surnaming();

Var is not recommened in ES6 :

Var variable are not recommened due to its hoisting behaviour and global scope declaration . 'var' variable are hoisted to the top of the scope , which can lead to unexpected behaivour also it can be redeclared with the same scope .

Block and Function scope in Javascript:

Block and function scope are both concepts related to variable visiblity and accessbility in javascript.

Function Scope:

JavaScript has function scope: Each function creates a new scope.Variables defined inside a function are not accessible (visible) from outside the function.Variables declared with var, let and const are quite similar when declared inside a function.They all have Function Scope:

Block Scope:

Block scope refers to the visiblity of variables declared within a block of code , such as within loops, if statements , or any code block enclosed within curly braces {}. IN js let and const have block scope. meaning they are accessible with in the block in which they are defined.

Hoisting of Variable :

In Javascript , hosting refers to the process where variable and function declaration are moved to the top of their containing scope during the compilation phase , regardless of where they are declared within that scope . This means that if you even declare a function later in your code. it behaves as if it were declared at the top.

It is important to note that only the declaration are hoisted not the intializations . For variables declared with var , the intialization is also hoisted . but for variable declared with let or const , only the declaration is hoisted , and the intialization remains in its original place.

console.log(name); // not working 
let name = 'alok tamrakar';
myFunction();

function myFunction(){
console.log(" hello , world "):
}

console.log(name);  // alok this is me 
var name = 'alok this is me ';