Important Javascript Questions and Answer

  • By Pooja Nandode-Bhavsar
  • May 29, 2024
  • JavaScript
Important Javascript Questions and Answer

Important Javascript Questions and Answer

Discover the most Important JavaScript Questions and Answers. Enhance your coding skills and interview readiness with this essential guide to JS fundamentals.

1. What is JavaScript? What is the role of JavaScript engine?

JavaScript is a programming language that is used for converting static web pages to

interactive and dynamic web pages.

A JavaScript engine is a program present in web browsers that executes JavaScript code.

JavaScript Engines

V8 (Chrome)

Spider-Monkey (Firefox)

Javascript-Core (Safari)

Chakra (Edge)

 

2. What are Client side and Server side?

A client is a device, application, or software component that requestsand consumes services

or resources from a server.

A server is a device, computer, or software application that provides services, resources, or

functions to clients.

 

3. What are variables? What is the difference between var, let, and const?

Using the flexible and popular programming language JavaScript, programmers may create

dynamic and interactive online apps and web pages. The ability to store and change data by

using variables is one of the fundamental ideas in JavaScript. Var, let, and const are the three

main ways to declare variables in JavaScript or we can say that these are the key words. Each

of these keywords has a certain purpose and has a different scope. The differences between

var, let, and const in JavaScript with examples are :-

Var:

From the starting of JavaScript, var has been the most traditional method of declaring

variables. Var-declared variables are function-scoped or we can say that var is function

scope, which means their scope is constrained to the function in which they are specified. It

will still be possible to access a variable that is declared using the var keyword inside of a

block, such as a loop or conditional expression.

function mine() {

if (true) {

var x = 10;

}

console.log(x); // Outputs 10

 

}

mine();

In this example, x is declared using var inside the if block, but it is still accessible outside the

block.

 

let:

let which was added in ECMAScript 6 (ES6), it follow block-scoped method of declaring

variables. Because of this, let is especially helpful for prevent variable hoisting and in scope-

related errors.

function mine() {

if (true) {

let y = 20;

}

console.log(y);

}

mine();

In this example, y is declared using let inside the if block, and it is not accessible outside the

block.

const:

Constants (const), when we use const keyword with variables then their initial value cannot

be changed. Const keyword also come in ES6. const is thus a good choice for declaring

variables that shouldn’t change while a block of code is being executed.

function mine() {

const z = 30;

z = 40;

}

mine();

In this example, z is declared using const and is immediately assigned the value 30. When

we attempt to change its value to 40, a TypeError is thrown because const variables cannot

be reassigned.

 

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4. What are some important string operations in JS?

Concatenation with concat()

Slicing with slice()

Substring Extraction with substring()

Substring Extraction with substr()

replace()

search()

toLocalLowerCase()

toLocalUpperCase()

indexOf()

includes()

lastIndexOf()

charCodeAt()

trim()

charAt()

valurOf()

split()

toString()

match()

 

//add multiple string

let str1 = "This"

let str2 = "Good"

let result = str1 + " " + str2

console.log(result);

//output: This Good

 

//using concat methods

let str1 = "This"

let str2 = "Good"

let result = str1.concat(" " , str2)

console.log(result);

//output: This Good

 

//using substring methods

let str1 = "This"

let str2 = "Good"

let result = str1 + " " + str2

let substring = result.substring(5,11)

console.log(substring);

//output: Good

 

//using length methods

let str1 = "This"

let str2 = "Good"

let result = str1 + " " + str2

let substring = result.substring(5,11)

console.log(substring.length);

//output: 4

 

//using toLocaleUpperCase methods

let str1 = "This"

let str2 = "Good"

let result = str1 + " " + str2

console.log(result.toLocaleUpperCase());

//output: THIS GOOD

 

//using toLocaleLowerCase methods

let str1 = "This"

let str2 = "Good"

let result = str1 + " " + str2

console.log(result.toLocaleLowerCase());

//output: this good

 

//using split methods

let str1 = "This"

let str2 = "Good"

let result = str1 + " " + str2

let arr = result.split(" ")

console.log(arr);

//output: [ 'This', 'Good' ]

 

//using replace methods

let str1 = "This"

let str2 = "Good"

let result = str1 + " " + str2

console.log(result.replace("This", "Mine"));

//output: Mine Good

 

//using replace methods

let str1 = " This Good "

console.log(str1);

console.log(str1.length);

let trim = str1.trim();

console.log(trim);

console.log(trim.length);

//output: This Good

 

//15

//This Good

//9

 

5. What is DOM? What is the difference between HTML and DOM?

 

The DOM(Document Object Model) represents the web page as a tree-like structure that allows JavaScript to dynamically access and manipulate the content and structure of a web

page.

In other words, a website made up of an HTML and CSS document. The browser creates a

representation of that document known as Document Object Model (DOM). This document

enables Javascript to access and manipulate the elements and styles of a website and also add

functionality to our page. So by this we can say that DOM have power to change our

website.

So HTML is the document and DOM have power to manipulate this document by using JS.

 

6. What is array? Explain different methods present in arrays in javascript?

 

Array is a collection of elements of the same datatype. Basically, array is 

 

1.push()   => adding element at the end of the array

                      //0   1         2     3     4    5

               let x=[4,”hello”,27.2,true,82,2];

  

               x.push(“welcome”);   

               document.write(“<br>after adding element at the end of the  array : “+x);

              

Output:

                    [4,”hello”,27.2,true,82,2,”welcome”];

 

2. unshift()=> adding element at first position of the array

 

                       //0   1         2     3     4    5

               let x=[4,”hello”,27.2,true,82, 2];

  

 

                  x.unshift(100);                                    //shift()

                  document.write(“<br>after adding element at the start  array : “+x);

 

Output:

                    [100,4,”hello”,27.2,true,82,2];

 

3. Splice => adding element at middle position of the array

 

                       //0   1         2     3     4    5

               let x=[4,”hello”,27.2,true,82, 2];

  

 

               x.splice( 2 ,0,”js”);      //here,2 is the index postion on whcih we want add an element “js”

                                     //the second argument is 0 which indicates it will not delete the element which is already positioned on index 2

               //x.splice( 2 ,0,”js”,”session”);

                document.write(“<br>after adding element at the middle of the  array : “+x);

 

4. concat()

               let x=[3,6,7,3,7,9];

                let y=[“raj”,”riya”,”jay”,true];

 

                           document.write(“<br>after x concat y  = “+x.concat(y));

      

Output:

after x concat y = 3,6,7,3,7,9,7,8,99,hello,78.5

 

5.filter()=> filter function is used to filter an array based on given predicate/condition

 

   //WAP to findout the elements of an array having length equals to 3

             let data=[“ram”,”hello”,”welcome”,”jay”,”run”,”program”];

 

             document.write(“original array = “+data);

                     

        //In below code, (p) => p.length == 3 ) is called as “arrow function”

         //filter function is used to filter an array based on given predicate/condition

        //here,in below case filter function takes one one value at a time from array “data” and   //store it into variable “p”

 

 document.write(“<br>after fiter data = “+data.filter((p) => p.length == 3 ));

 

6. indexOf and lastIndexOf => is used to findout the index of first and last occurrence of given element respectively

 

                  //0  1   2        3 4     5      6   7   8     9

          let x=[3,23,”hello”,7,3,”hello”,9,12,”hello”,99];

 

           document.write(“<br>array x = “+x);

 

            //indexOf()=> method findout FIRST occurrence of given element from the array

           document.write(“<br>first index of hello is  = “+x.indexOf(“hello”));

 

            //lastIndexOf()=> method findout LAST occurrence of given element from the array

        document.write(“<br>last index of hello is  = “+x.lastIndexOf(“hello”));  

 

          //indexOf()=> method findout MIDDLE occurrence of given element from the array

          document.write(“<br>middle index of hello is  = “+x.indexOf(“hello”,3));  //findingout indexof hello after 3rd position

 

  Output:

array x = 3,23,hello,7,3,hello,9,12,hello,99

first index of hello is = 2

last index of hello is = 8

middle index of hello is = 5

 

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Author:-

Pooja Nandode-Bhavsar
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