Know Most Common JavaScript Interview Questions With Answers

The Java Script helps the web page to be more interactive than a regular static page. In other words, JavaScript is added to give a behavior to the web page. In this article, we are going to look at few very important javascript interview questions.

Javascript Interview Questions?

Following are the javascript interview questions and answers:

  1. What is the difference between NULL vs. UNDEFINE

Ans:- When we use NULL for a variable, we can not use it because it has been defined as a NULL. In contrast, UNDEFINE means that a variable can be used any time for any Data Type.

E.g. var myValue = null;

console.log(myValue);

Output:

null

2. What is NaN?

Ans:- Well, NaN is a global object property. It is also known as a variable in the global scope. NaN stands for Not-A-Number.

E.g., var myValue = “Hello” – “World”;

console.log(myValue);

Output:

NaN

3. What is different between == and === ?

Ans: In double ==, if we add a string and a number, the result will be shown as true.

E.g., 

var myNum1 = “10” ;

var myNum2 = 10;

console.log(typeof(myNum1));

console.log(typeof(myNum2);

console.log(myNum1 == myNum2);

Output:

String

Number

true

Now, if we use triple ===, we will be getting the answer as false because, in triple ===, it also checks the Data Type.

E.g., 

var myNum1 = “10” ;

var myNum2 = 10;

console.log(typeof(myNum1));

console.log(typeof(myNum2);

console.log(myNum1 === myNum2);

Output:

String

Number

false

Also Read: How Do I Start JavaScript Programming? Get Examples

4. How can I tell the distinction between Function Parameters as well as Function Arguments?

Answer:- Function Parameters are the name which is defined in the listed Functions. Function Arguments represent the actual value that is passed to the program.

5. Why do we use Function in JavaScript?

Answer:- The function helps to reuse the code. We have to define the code ones and reuse them again.

6. How to write a table in JavaScript?

Answer:-

for(var table = 1; table <= 10; table++ ){

    var tableof = 8;

    console.log(tableof + “*” + table + “=” + tableof * table );

}

OutPut:- 

8*1=8

8*2=16

8*3=24

8*4=32

8*5=40

8*6=48

8*7=56

8*8=64

8*9=72

8*10=80

JavaScript Interview Questions
The Java Script helps the web page to be more interactive than a regular static page.

7. What is Hoisting in JavaScript?

Ans: Hoisting is a default behavior that takes all the variable (var) at the top.

function newvar(){

  x = 10;

  console.log(x);

  var x;

}

8. What are the different kinds of data types available in JavaScript?

Ans:- There are many data types in Javascript, but a few which are used very commonly are String / Number / BigInt / Boolean / Undefined / Null / Symbol / Primitive types / Non-primitive types.

9. What is Higher Oder Function in JavaScript?

Ans: Higher Oder Function are at the top of all the function.

function oderFunction() {

  return function() {

    return “Learn JavaScript”;

  }

}

var y = oderFunction();

y()

Output:- Learn JavaScript

10. What is call(), apply() &, bind() methods in JavaScript?

Ans: Javascript has these methods redefine in it.

Call Methods:-

function newCall(){

  return “Hello ” + this.name;

}

var obj = {name: “World”};

sayHello.call(obj);

Output: “Hello World”

Apply Method:-

function newApply(message){

  return this.name + ” is ” + message;

}

var people = {name: “Amit”};

newApply.apply(people, [“Sing”]);

Bind Method:- 

fn.bind(thisArg[, arg1[, arg2[, …]]])

11. Explain callback in JavaScript?

Ans:- In JavaScript, the callback is a function that will be executed only after another function gets executed.

function firstNum(sum){

  console.log(Math.floor(sum / 3));

}

function mulBynum(sum){

  console.log(sum * 3);

}

function operationOnSum(num1,num2,operation){

  var sum = num1 + num2;

  operation(sum);

}

operationOnSum(3, 3, firstNum);

operationOnSum(3, 3, mulBynum);

12. What is DOM in JavaScript? (This is a very important javascript interview question that is asked even in React interviews)

Ans:- JavaScript, DOM stands for Document Object Model. DOM is an interface to HTML along with XML documents. When we call an HTML code on the server, it creates a DOM, and then we can use many elements which are use full for the website.

13. Explain the arrow function?

Ans:- The arrow function was introduced in the year 2016 in ES6. With the help of the arrow function, we can write shorter and new syntax in JavaScript.

14. Explain Object Destructuring in JavaScript?

Ans:- In JavaScript, the concept of object destructuring is an innovative method of extracting elements from objects or arrays. It can only be observed within the ES6 Version.

15. What are the Disadvantages of JavaScript?

Ans:- There is no support for multithreading and multiprocessing. Reading and writing files are not allowed in javascript programming. There is no support for networking applications in JavaScript.

16. What is the Bug in JavaScript interview questions?

When we subtract an Int from a string, it shows us a correct answer which is not valid according to the logic, and we cant Subtract a number from a word.

E.g. console.log(25 – “15”); // Bug

Output: 10

2. If we try o find the value of NULL, we get the answer as an object, which is incorrect because NULL can not be an object.

E.g. var myValue = null;

console.log(myValue);

console.log(typeof(myValue));

Output:

null

object

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