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By Umesh Singh
Last update: 20 Jun 2020, 28 Questions
Introduction to JavaScript
JavaScript interview questions

JavaScript is a high-level programming language that is known for its dynamic, prototype-based and weakly typed characteristics. It conforms to the ECMAScript specification and is one of the core technologies of the World Wide Web. Initially only implemented client-side, JavaScript is now embedded in other types of software, including server-side in web servers and non-web programs such as PDF software and word processors. This information is often asked in JavaScript interview questions.

Advantages of JavaScript

  • Client-side JavaScript is quite fast.
  • The syntax is simple and flexible.
  • It develops well with most of the other popular languages.
  • Drag and drop components give a rich interface to your site.
  • Third party add-ons enable developers to write rich snippets.

Most Frequently Asked JavaScript interview questions

1. What is JavaScript and why it is used?

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.

2. What are the difference between undefined and not defined in JavaScript?
  undefined Not defined
1. A variable was declared with a keyword "var" but not assigned with a value. A variable have not been declared without assignment:
2. Example :
var a = 1, b;
console.log(a) //1
console.log(b) //undefined
Example :
var a = 1, b;
console.log(f) // not defined
3. What are the data types available in JavaScript?

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.

4. How to create an array in JavaScript?

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.

5. How to create an object in Javascript?

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");
6. What is the difference between "==" and "==="?

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

S.no "==" "==="
1. The operator "==" tests the abstract equality, which means before making equality comparison, it does the necessary type conversions. The operator "===" tests only strict equality, i.e., this will not perform type conversion; hence if values compared here are not the same type, it will return false.
7. What is the difference between "var" and "let" Keywords?

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

  let var
1. It can be only available inside the scope it's declared, like in for loop. It can be accessed outside the loop.
2. It is block scoped. It is function scoped
8. Is JavaScript client side or server side?

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.

9. What is NULL in Javascript?

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.

10. What is a closure JavaScript?

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
11. What is console.log() and why it is used?

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.

Example

                                                    

$('#form').submit(function() {
     console.log(‘Your form is submitted succesfully!’);
     // do something
});

12. How do you write a comment in Javascript?

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 */.

13. Explain the difference between call() and apply()?
S.no Call() Apply()
1. In JavaScript, call() is a predefined method used to invoke (call) methods with the use of an owner object as an argument (parameter). apply() is a method to write another way which can be used on different objects. It’s usually action similar to call() action.

This fundamental difference can is asked in many JavaScript Interview Questions and Answers from both freshers and experienced.

14. What is the importance of JavaScript?

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.

15. What are the difference unescape() and escape() functions?
S.no unescape() escape()
1. The unescape() function uses a string as a parameter, and used to decode that string encoded by the escape() function. The escape() function in JavaScript is used to encode a string. It makes a string portable so that it can be transmitted across any network to any computer with ASCII character support.
16. What is the use of decodeURI() and encodeURI()?
S.no decodeURI() encodeURI()
1. The decodeURI() function works reverse to the encodeURI(), taking an encoded string and then replacing the tokens with the regular characters. encodeURI() is used to pass the string value that the user requires to convert. This function returns the URL encoded value.
17. What is NaN?

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.

18. What is the difference between Arrow function and normal function?
S.no Arrow function Normal function
1. Introduced in ES6, it provides a concise way to write functions in the JavaScript. The context present inside arrow functions is statically or lexically defined. It’s a procedure to produce a set of statements that calculate a value or performs a task. We have to define a function in the scope from which we wish to call it, to use it.
19. What is singleton class in JavaScript? Explain

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.

20. Explain export & import in JavaScript?
S.no Export Import
1. In JavaScript, the export statement is used when creating modules for JavaScript to export functions, primitive values or objects from the blades so they can be used by the programs with import statement which works opposite to it. The import statement is used when we import buildings which are exported by another module. All imported modules are in strict mode. This statement can’t be used in embedded scripts.
21. What is design pattern in JavaScript? Explain

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

22. How to create a function in Javascript?

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:

Example

                                                    

function myFunction(p1, p2) {

return p1 * p2; // The function returns the product of p1 and p2

}

23. How can you do a character count in Javascript?

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

var str = "Hello World!";

var n = str.length;

24. How to clone an object in Javascript?
There are three types of cloning available in JavaScript.
  • Deep copy using iteration
  • Converting to JSON and back
  • Using Object.assign
26. How we can add title of the page by JavaScript?

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

document.title = 'Your desired title';

27. What are the difference between JavaScript and jQuery?
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.

Development History of JavaScript

Invented by Brendan Eich in 1995, JavaScript became an ECMA standard in 1997. JavaScript was first to run on the Netscape 2 browser.

Latest Version: ECMAScript 2018 or ES9 is the latest version and was released in October 2018.