JavaScript Interview Questions and Answers

JavaScript is a high-level programming language that is used to calculate, manipulate and validate data. With the help of JavaScript, we can update and change both HTML and CSS.

We can write JavaScript code in an HTML page. When a HTML page loaded with JavaScript code, the script is sent to the browser for manipulate then HTML or CSS code will be rendered. It is a client-side technology.

In JavaScript data types is an important concept that helps to operate on variables,JavaScript supports mainly two different kinds of data types-

• Primitives
• Objects

1. Primitives Data Type

It supports six data types that’s are given below.

• Boolean: It represents only true or false based on condition.
• Number: It can be written with or without decimals. Example var num1 = 32;
• String: It is used to store texts and inside of either double or single quotes. Example var str_one = 'Best Interview Question';
• Null: It is "nothing" but supposed to be something that does not exist.
• Undefined: It has no value but variable is exists. Example var MyVariable_1;
console.log(MyVariable_1); // undefined
• Symbol: It is new in ES6. It is an immutable data type that have unique value.Example : const BestInterviewQuestion = Symbol(‘BestInterviewQuestion’);

2. Objects Data Type

It is a type of not primitive data Type. It is a collection of properties and these properties are stored in key and value pairs.

In JavaScript, arrays is used to store multiple values within a single variable at a time.

Syntax for creating Arrays in JavaScript:

var arrayName = [value1, value2, value3];

Example:

var name = [‘best’,’interview’,’questions’];

We can access these values with:

var name1 = name[0]; console.log(name1); // it with display best.

It is not a jQuery feature but a feature for debugging purposes used by developers. It is used to writes a message to the console.

console.log() accepts a parameter which can be an object, an array or any message.

Syntax : console.log(name); // here name is object, an array or variable.

There are various ways of creating an Object in JavaScript.

• With the Object():
  Example : var name = new Object();
• With Object.create():
  Example : var name = Object.create(null);
• With the bracket syntactig sugar:
  Example : var name = {};
• With a function constructor:
  Example :
  var ObjectName = function(Vname) {
    this. Vname = Vname
}
var NewVar = new ObjectName("BestInterviewQuestion.com");
• With function constructor + prototype:
  Example :
  function myObj(){};
myObj.prototype.name = "bestinterviewquestion.com";
var Newk = new myObj();
• With ES6 class:
  Example :
  class myObject {
    constructor(name) {
        this.name = name;
    }
}
var e = new myObject("bestinterviewquestion.com");

Both are used for function declaration in JavaScript but the difference are given below :-

The difference between "==" and "===" is as following

Designed by Brendan Eich in May 1995 and developed under Netscape Communications, the

JavaScript was first released in September 1995 with the name Netscape Navigator 2.0 which was

later renamed to JavaScript. Interestingly, Eich wrote the full prototype of JavaScript in just 10 days.

JavaScript is a client-side language it gets executed at the user side. Whenever we browse the web, all the files from JavaScript are first fetched from the server and then performed at our side by the browser.

There are two types of comments we can write in JavaScript; single line comment and multiline comment. To create a single line comment, the users have to place two slashes “//” in front of text or code which they wish to have the JavaScript interpreter ignore. For multiline comments, use /* from the beginning of the comment and end it with */.

In JavaScript, Null represents the absence of any object value. If a user tries to reference a null variable, he/she will get an error. It implies to no string, no number, no valid Boolean value, and no array object.

The 'use strict' is a new ECMAScript version 5 directive. It’s a literal expression, not a statement, ignored by previous JavaScript versions. It is purposed to indicate that the code should be executed in the "strict mode."

Not-a-Number or NaN is a weirdo Global Object present in JavaScript and frequently returned when failed mathematical operation happened. We can use global function isNaN() to find out if the variable is a NaN.

It’s a JavaScript feature where an inner function has access to the enclosing or outer function’s variables, which is defined as a scope chain.

There are three types of scope chains:

• Access to its own scope
• Access to variables of outer functions
• Access to global variables

To count the number of elements in an array using JavaScript, use the following

Singleton is a JavaScript object which can be instantiated one time, whereas the singleton pattern is a design pattern that restricts the reinstallation of a class to one object. It allows only a single instance of itself to be created and can give access to that created instance.

Point to be noted:- Don’t let this JavaScript Interview Question take your job opportunity. Read it twice and then go for the interview.

It’s an advance object-oriented solution designed to solve commonly occurring software problems. These are reusable designs and interact with objects.

The javascript function is a block of code which can be designed according to the user to perform a particular task. A basic JavaScript function syntax is mentioned below:

To perform a character count in JavaScript, use the following syntax:

var str = "Hello World!";

var n = str.length;

There are three types of cloning available in JavaScript.

• Deep copy using iteration
• Converting to JSON and back
• Using Object.assign

To change the title of a web page using JavaScript, we have to use the following syntax:

document.title = 'Your desired title';

It is a client side technology, it is mainly used for gives client side validation, but it have lot of features which are given below;

• It is an object-based scripting language.
• It helps the user more control over the browser.
• JavaScript Handling dates and time also.
• It also helps to Detect the client's browser and Operating System.
• JavaScript is light weighted & case sensitive.
• All most JavaScript control statements syntax is same as syntax of control statements in C programming language.

These JavaScript Interview Questions can be beneficial in your interviews.

var currentLocation = window.location;

On the basis of functioning

JavaScript is a scripting language which is compatible with all the major browsers and used to create interactive web-based applications. While jQuery is a framework which is the fastest JavaScript library used for the simplification of HTML document.

Time and simplicity

JavaScript is complex because it consumes the time of developers as it is important for the developers if they are using JavaScript then they have to create their own script. In the case of jQuery, developers need not write scripting as it already exists in the libraries.

Animation support

With JavaScript, it cannot be possible for the developer to use animation. On the other hand, jQuery allows the developers to use unbuild animations to make the applications interactive and attractive.

Featural aspects

JavaScript programming language is a combination of the European Manufacturing Association Script (ECMA) and Document Object Model (DOM). While on the other hand jQuery has a Document Object Model (DOM).

Interactivity feature

With the use of JavaScript, developers can make web pages more useful by suppling immediate feedback. While in case of jQuery developer can create web-based applications which becomes easier for the developer to customize the application as per the user's requirement.

With JavaScript, the user can declare variables with three keywords, which are let, var, and const. The terms var let and const are somewhat confusing. We will explain all 3 variables with the help of examples.

The difference between the variables var, let as well as const is described below in different ways.

• Scope
• Hoisting
• Reassign the value
• Redeclaration of the variable

NOTE: If you want to read more about the difference between var, let as well as const then you can visit here.

The main differences between arrow function and normal function are based on some parameters like.

• Syntax
• Arguments that are binding.
• "this" keyword is used to describe the use.
• Utilizing a keyword that is new.
• No duplicates of identified parameters.

Syntax of normal functions

// Function declaration
function printHello(name) {
   return `Hey ${name}`;
}

NOTE: If you want to read more about Regular vs Arrow functions then you can visit here.

// Function expression
const printHello = function(name) {
  return `Hey ${name}`;
}

Syntax of arrow functions

const printHello = (name) => `Hey ${name}`;

1. Using includes()

const fruits = ["Banana", "Orange", "Apple", "Mango"];
console.log(fruits.includes("Mango"));

2. Using indexof()

const fruits = ["Banana", "Orange", "Apple", "Mango"];
if(fruits.indexOf("Orange") !== -1) {  
   alert("Yes, the value exists!")  
} else {
   alert("No, the value is absent.")  
}

This is a type of -  object
This is a type of -  object

typeof

Example

var x = 12345; 
console.log(typeof x)
x = 'string'; 
console.log(typeof x)
x = { key: 'value' };
console.log(typeof x)

Answer

number
string
object

2

It will give an error like "TypeError: Assignment to constant variable".

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