JavaScript Top 10 Interview Questions

1- What is difference between var and let ?

Ans: i) var is in Javascript since begining and let is introduced in ES2015/ ES6

        ii) let has block scope and var has function scope

                e.g:  

              function myfun() {

                    if (true) {

                         var a = 2;

                          let b = 1;

                    }

                    console.log(a); //2

                    console.log(b);  // b is not defined

              }

        iii) var get hoisted to it's top of it's function

        e.g:   

                function myfun() {

                    if (true) {

                        console.log(a); // undefined 

                        console.log(b);  // b is not defined

                         var a = 2;

                          let b = 1;

                  }

              }

2- what is difference between let and const

ans:  const will not let you to reassign a value but you can modify it

        let will let you do the reassign value as well as modification to value 

3- what is difference between undefied and null

ans : Both represent empty value but when we define variable and not assign value to it then javascript put it is a undefined and 

        null we do it by assigning it manually we can also assign undefined but which is not a good idea

        typeof(undefined)  = => undefined

        typeof(null) ==> object

4- What is prototype inheritance in Javascript ?

ans : Every object of javscript has prototype property by default

This is why you can use a method like sort() on an array instance.

prototypical inheritance refers to the ability to access object properties from another object. We use a JavaScript prototype to add new properties and methods to an existing object constructor. We can then essentially tell our JS code to inherit properties from a prototype

5- What Closure in javascript ?

ans: A closure is a function having access to the parent scope, even after the parent function has closed

(we can use it for counter type of method)

let Obj = function () {

    let i = 0;

   return {

    setI(k) {

       i = k;

     }

    getI() {

       return i;

     }

    }

}

let x = Obj();

x.setI(2);

console.log(x.getI); // 2