JavaScript Interview Question Set 1

Categories: Java 8(JDK1.8)

How to write a comment in JavaScript?

There are two types of comments in JavaScript.

a) Single Line Comment: It is represented by // (double forward slash)

b) Multi-Line Comment: Slash represents it with asterisk symbol as /* write comment here */


How to create a function in JavaScript?

To create a function in JavaScript, follow the following syntax.

function function_name(){  

//function body  

}  


What are the different data types present in JavaScript?

There are two types of data types in JavaScript:

a) Primitive data types

b) Non- Primitive data types


 What is the difference between == and ===?

The == operator checks equality only whereas === checks equality, and data type, i.e., a value must be of the same type.


How to write HTML code dynamically using JavaScript?

The innerHTML property is used to write the HTML code using JavaScript dynamically. Let's see a simple example:

document.getElementById('mylocation').innerHTML="<h2>This is heading using JavaScript</h2>";   


 How to write normal text code using JavaScript dynamically?

The innerText property is used to write the simple text using JavaScript dynamically. Let's see a simple example:

document.getElementById('mylocation').innerText="This is text using JavaScript";   


How to create objects in JavaScript?

There are 3 ways to create an object in JavaScript.

a) By object literal

b) By creating an instance of Object

c) By Object Constructor

Let's see a simple code to create an object using object literal.

emp={id:102,name:"Rahul Kumar",salary:50000}   



How to create an array in JavaScript?

There are 3 ways to create an array in JavaScript.

a) By array literal

b) By creating an instance of Array

c) By using an Array constructor

Let's see a simple code to create an array using object literal.

var emp=["Shyam","Vimal","Ratan"];    



What does the isNaN() function?

The isNan() function returns true if the variable value is not a number. For example:

function number(num) {  

  if (isNaN(num)) {  

    return "Not a Number";  

  }  

  return "Number";  

}  

console.log(number('1000F'));  

// expected output: "Not a Number"  

  

console.log(number('1000'));  

// expected output: "Number"  


What is the output of 10+20+"30" in JavaScript?

3030 because 10+20 will be 30. If there is numeric value before and after +, it treats as binary + (arithmetic operator).

function display()  

{  

  document.writeln(10+20+"30");  

}  

display();  



What is the output of "10"+20+30 in JavaScript?

102030 because after a string all the + will be treated as string concatenation operator (not binary +).

function display()  

{  

  document.writeln("10"+20+30);  

}  

display();  

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