General JavaScript Interview Questions

 

1. What is JavaScript?

JavaScript is a programming language used both on the client-side and server-side that allows you to make web pages interactive.

It improves the user experience of the web page by converting it from a static page into an interactive one (dynamic page).

2. What is the use of a JavaScript?

Some of the uses of JavaScript are:

• Adds interactive behavior to web pages
• Creates web and mobile apps
• Builds web servers and develops server applications
• Game development, etc.

3. What is the use of a document in JavaScript?

JavaScript Document object is an object that provides access to all HTML elements of a document. It has various properties that refer to other objects which allow us to access and modify the document content.

4. What are the differences between JavaScript and Java?

JavaScript	Java
JavaScript is a scripting language	Java is a programming language
JavaScript code is run on a browser only	Java creates applications that run in a virtual machine or browser
JavaScript file has the file extension  .js	Java file has the file extension  .java

5. How to print in JS?

We can use the

console.log()

to print to JavaScript console. The JavaScript

console.log

method is mainly used for code debugging as it makes the JavaScript print the output to the console.

Example:

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console.log("Hello World!");

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6. What is execution context?

Execution Context

• The environment in which Code runs is called Execution Context.
• Execution context contains all the variables, objects, and functions.
• Execution Context is destroyed and recreated whenever we reload an Application.

7. How to display the output in JavaScript (not in the console)?

JavaScript can display data in different ways:

textContent

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let displayEl = document.getElementById("display");

displayEl.textContent = 10 + 15;

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innerHTML

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let displayEl = document.getElementById("display");

displayEl.innerHTML = 10 + 15;

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document.write()

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document.write(10 + 15);

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alert

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alert(10 + 15)

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8. List some of the advantages of JavaScript.

• Server interaction is less.
• Feedback to the visitors is immediate.
• Interactivity is high.

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

3030

10. What language is JavaScript?

JavaScript is known as the Scripting Language for Web pages.

It is used add interactivity in Webpages.

11. Where do you write JavaScript?

We can write JavaScript in the HTML

script

element.

The HTML

script

element contains script statements, or it points to an external script file through the HTML

src

attribute.

12. How to add JS to HTML?

There are three ways to add JavaScript to HTML pages.

• Embedding code
• Inline code
• External file

Embedding code: We can use the HTML

script

element to wrap around JavaScript code inside the HTML program.

Inline code: It can be used when we have to call a function in the HTML event attributes.

External file: We can also create a separate file to hold the code of JavaScript with the

.js

extension and later add it into our HTML document using the HTML

src

attribute of the HTML

script

element.

13. What is the solution to the absence of an HTML script element without an HTML src attribute?

The solution to the absence of an HTML

script

element without an HTML

src

attribute is to embed the JavaScript code in the HTML document using HTML

script

element.

Example

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<script type="text/javascript">

let buttonEl = document.getElementById("button");

buttonEl.addEventListener("click", function (event) {

console.log("button is clicked");

});

</script>

HTML

14. Who created JavaScript?

The JavaScript was created by Brendan Eich at Netscape.

15. How to convert a string to a number?

We can convert a string to a number using the

parseInt

method.

16. What is an alert method in JS?

The alert methods instructs the browser to display a dialog with an optional message, and to wait until the user dismisses the dialog.

Example:

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alert("Hello")

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17. How do you debug errors in JavaScript?

• Using

  console.log()
  Method

• Setting Breakpoints or using Debugger in code

• Using Browser Debugging tools

Note

To know more about debugging errors, refer to below references.

• Reference 1
• Reference 2

18. How can we submit a form in js?

We can submit the form,

• By clicking the HTML 
  button
   element with type submit.
• By clicking the HTML 
  input
   element with type submit.

19. How to implement the form validation in JavaScript?

You can refer to this session code for implementing form validation in JavaScript.

20. How many scopes of a variable does JavaScript have?

JavaScript Variable has two scopes, local and global.

Local Scope:

If a variable is declared with

const

or

let

within a curly brace (

{}

), then it is said to be defined in the Block Scope.

Example:

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function isEven(){

let number = 2;

return number%2 === 0

}

isEven()

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Global Scope:

If a variable is declared outside all functions and curly braces (

{}

), then it is said to be defined in the Global Scope.

Example:

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let number = 2;

function isEven(){

return number%2 === 0

}

isEven()

JAVASCRIPT

21. What are functions hoisting and variables hoisting?

Hoisting is a JavaScript technique that moves variables and function declarations to the top of their scope before code execution begins.

Example:

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console.log(functionBelow("Hello"));

function functionBelow(greet) {

return `${greet} world`;

}

console.log(functionBelow("Hi"));

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Output:

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Hello world

Hi world

Even though the function declaration is after the function call, the function is called. This is possible due to function hoisting.

22. What are the differences between class and object?

Class	object
Class is a template for creating objects in a program	Object is an instance of a class
Class is a logical entity	Object is a physical entity
Class does not allocate memory space	Object allocates memory space
You can declare class only once	You can create more than one object using a class
Classes can't be manipulated	objects can be manipulated
Classes doesn't have any Values	Objects have its own values
You can create a class using the "class" keyword	You can create an object using the "new" keyword in JavaScript

23. Define class and object?

Class:

The Class is a special type of function used for creating multiple objects.

Syntax:

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class ClassName {

constructor() {}

method_1() {}

method_2() {}

method_3() {}

}

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Object:

• An Object is a collection of properties.
• A property is an association between a name (or key) and a value.
• For example, a person has a name, age, city, etc. These are the properties of the person.

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let person = {

firstName: "Rahul",

lastName: "Attuluri",

age: 28,

};

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24. What is Class Inheritance in JavaScript?

Inheritance is a mechanism by which a class inherits methods and properties from another class.

Syntax:

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class SuperClass {

constructor(property1) {

this.property1 = property1;

}

method1() {}

}

class SubClass extends SuperClass {

constructor(property1, property2) {

super(property1);

this.property2 = property2;

}

method1() {}

}

let myObject = new SubClass(property1, property2);

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Collapse

Here,

SubClass

inherits methods and properties from a

SuperClass

.

• The

  extends
  keyword is used to inherit the methods and properties of the superclass.

• Calling

  super()
  makes sure that SuperClass constructor() gets called and initializes the instance.