Common JavaScript Interview Questions for Web Developers

WHAT IS JAVASCRIPT ?

JavaScript is one of the client side scripting language, which improves the user experience of webpage by converting it from a static page into an interactive one.

ADVANTAGES OF JAVASCRIPT ?

improves user-experience of web page.
client side execution.
can validate on browser.
easy language to learn or work with.

DIFFERENCE BETWEEN JAVA AND JAVASCRIPT ?

Java is an object-oriented programming language but the JavaScript is the client-side scripting language, which can be included inside the HTML pages.

WAYS OF INTRODUCING JAVASCRIPT IN HTML PAGES ?

There are mainly 2 ways by which we can include the JavaScript inside HTML and these ways are as follows:-

Internal:- Here in the same html page inside the head tag (<head>) we can write our JavaScript by following under the <script> tag.
External:- Here we need to write our JavaScript code in any other page, which must need to save by the .js extension and which must be linked inside the <head> tag or below body close tag </body> of the html page.

CAN WE EMBED TWO JAVASCRIPT FILES, IF YES THEN HOW ?

Yes, we can able to embed two JavaScript files at the same time. We need to simply include both page inside script tag.

For example:- <script src=”main1.js” ></script> and <script src=”main2.js”></script>

WHAT IS THE USE OF DOCUMENT.WRITE() FUNCTION N JAVASCRIPT ?

Through this document.write() function, we generally use to display our data. And it usually override all the other elements or tags or sections presented in the HTML page.

WHAT IS WINDOW.ALERT() FUNCTION OR ALERT() FUNCTION IN JAVASCRIPT ?

window.alert() or alert() function both are the same term, actually alert() function is the window object only, so there is no difference whether you write window.alert() or alert(), both will be performing same task.

WHAT ARE THE ALERTS USED FOR IN JAVASCRIPT ?

Alerts are basically any pop-up or dialogue box, which can store any important information or data etc. to grab the high attention from the users.

HOW CAN WE DEFINE THE VARIABLES IN JAVASCRIPT ?

There are three reserved keywords are in JavaScript, by which we can define any variable and these are as follows:-

let
var
const

NAME DIFFERENT TYPES OF OPERATORS IN JAVASCRIPT ?

There are mainly 5 types of JavaScript operators which are as follows:-

Arithmetic Operators
Relational (Comparison operators)
Logical Operators
Bitwise Operators
Assignment Operators

NAME THE PRIMITIVE DATA TYPES IN JAVASCRIPT ?

Mainly there are 6 types of primitive data types, which are as follows:-

undefined
Boolean
Number
String
BigInt
Symbol

WHAT WE CAN ACHIEVE THROUGH TYPEOF OPERATOR IN JAVASCRIPT ?

We can able to find any data type of any variable, or etc. through typeof operator in JavaScript.

NAME THE NON-PRIMITIVE DATA TYPES IN JAVASCRIPT ?

There are mainly 3 non-primitive data types in JavaScript and these are as follows:-

Object
Array
RegExp

DIFFERENCE BETWEEN UNDEFINED AND NULL OBJECT IN JAVASCRIPT ?

The main difference is of data type. When we use to apply the typeof operator with null, then it tells us that null is an object whereas when we apply the typeof operator with undefined, then it tells us that undefined is undefined.

WHAT IS THE USE OF ISNAN() METHOD IN JAVASCRIPT ?

isNaN() method is use to identify whether any variable or etc. is Number or not. The output will come true only if particular variable or etc. is not a number else the output will come false.

WHAT ARE THE 6 FALSY VALUES IN JAVASCRIPT ?

0
" "
undefined
null
NaN
false

WHAT ARE LOOPS USED FOR IN JAVASCRIPT ?

If we want any block of code to execute number of times then we need the loops here.

HOW MANY DIFFERENT KINDS OF LOOPS DOES JAVASCRIPT SUPPORT ?

There are mainly 5 kinds of loops in JavaScript, and these are as follows:-

for loop:- loops any block of code.
for/in loop:- loops through properties of an object.
for/of loop:- loops through values of an object.
while loop:- loops through block of code when specified or particular condition be true.
do/while loop:- loops through block of code when specified or particular condition be true.

WHY WE NEED FUNCTION IN JAVASCRIPT ?

we can reuse our code.
can define code once and use it several times.
can also include functions with different arguments.
can call the functions very easily and from anywhere in the same project.

WHAT DO YOU MEAN BY FUNCTION EXPRESSION ?

When we use to assign any function in any sort of variable, then it is known as the function expression.

For example:- var funExp = sum(5,10).

WHAT DO YOU MEAN BY ANONYMOUS FUNCTION IN JAVASCRIPT ?

This is that type of function, where it is not necessary to have the function name. It also use to shortens the code.

WHAT ARE TEMPLATE LITERALS IN JAVASCRIPT ?

This is introduced in ECMAScript6 (2015), and it’s been very easy and interesting way to express the logics. We need to write our logics here inside the backticks (` `).

WHAT ARE ARRAYS USE FOR ?

Arrays in JavaScript use to hold as many values inside any single variable, and simultaneously it can hold as many values, without taking care of their data types.

DIFFERENCE BETWEEN FOREACH() AND MAP() METHOD IN JAVASCRIPT ?

ForEach() method of JavaScript always return undefined whereas map() method of JavaScript returns new array.
We can’t able to add reduce() or sort() method etc. in ForEach() method of JavaScript whereas in map() method we can able to add other methods like sort() or reduce() method.

WHAT IS THE USE OF SETTIMEOUT() METHOD IN JAVASCRIPT ?

To perform any task or to execute any type of code after a specified time intervals, then we need this setTimeout() method of JavaScript.

WHAT IS THE USE OF ADDEVENTLISTENER IN JAVASCRIPT ?

We use to add the events in our elements through this addEventListener() method. It is been introduced in ECMAScript 2015. In simple words, we can say it attaches an event handler to the document.

WHAT IS 'THIS' KEYWORD IN JAVASCRIPT ?

'This' keyword refers to the object where it was called. It generally refers to that object it belongs to.

CAN YOU SUBMIT THE FORM USING JAVASCRIPT, IF YES THEN HOW ?

Yes, we can able to submit the form using the JavaScript. And this we can achieve through:- document.form[0].submit();

WHICH IS FASTER JAVASCRIPT OR ASP SCRIPT ?

Of course, here the JavaScript wins as it is the client-side scripting language and it also does not need any assistance of web server to execute.

HOW CAN I ADD CLASS TO ANY ELEMENT USING JAVASCRIPT ?

For adding the class in element through JavaScript, we can use classList.add() method.

For example:- document.queryselector(“#demo”).classList.add(“newclass”);

WHAT IS AN IMMEDIATELY INVOKED FUNCTION IN JAVASCRIPT?

An Immediately Invoked Function is a function which runs as sson as it is defined. We need to add two sets of parentheses in the last where the function we declare.

HIGHER ORDER FUNCTIONS IN JAVASCRIPT ?

That JavaScript function which either takes a function as an argument or returns a function, means they can easily

CURRYING IN JAVASCRIPT ?

A technique, we can say an advanced technique to transform a function of arguments n, to n functions of one or less arguments.

DIFFERENT TYPES OF SCOPES IN JAVASCRIPT ?

There are three types of scopes in JavaScript and these are as follows:-

Global Scope
Block Scope
Local or Function Scope

WHAT IS A FUNCTION SCOPE ?

Variables or functions that are declared inside a function have local or function scope, that means we can't able to access those variables or functions outside from the function.

WHAT IS A GLOBAL SCOPE ?

Variables or functions that are declared in the global namespace have a global scope, that means we can able to access those variables or functions outside or inside or from anywhere inside the code.

WHAT ARE CLOSURES ?

An ability of a function to remember the variables and functions that are declared in its outer scope or functions to store a variable for further reference even after it is executed.

WHAT IS CALLBACK HELL ?

An anti-pattern with multiple nested callbacks which makes code hard to read and debug when dealing with ansynchronus logic.

WHAT IS FIRST CLASS FUNCTION ?

In JavaScript, functions are the first class objects. It basically mean when functions are treated like any other variable, like when functions are passed as an argument.

var	let
It has function scope.	It has block scope.
Variables are been hoisted here.	Hoisted but not initialized.
From the beginning it is available.	Introduced in ES6.

LIST THE BENEFITS OF USING MODULES ?

Reusability
Namespacing
Maintainability

WHY COOKIES ARE NEEDED ?

As cookie use to store the user profile details as soon as any user visits the page. And through it, can able to use the information about the user.

WHAT IS STRICT MODE IN JAVASCRIPT ?

Strict mode is a new advanced feature in ECMASCRIPT 5 that allows you to place a program, or a function, in a "strict" operating context. Sincerly you need to place your JavaScript code here.

WHAT IS THE USE OF typeof OPERATOR ?

We can able to find the data type of JavaScript variable. Means, it simply returns the type of a variable or an expression.

WHAT IS THE USE OF isFinite FUNCTION ?

isFinite() function is used to determine whether a number is a finite, legal number. It returns 'true' only if the numner is finite without decimal and positive or negative infinity.

WHAT IS THE EVENT CAPTURING ?

It is a type of event propogation where the event is first captured by the outermost element, and then it triggers on the descendants or children of the targeted element in the same nesting hierarchy.

RELATION BETWEEN JAVA AND JAVASCRIPT ?

Java and JavaScript are totally two different programming languages. Although both languages are Object Oriented Programming languages and have similar basic features(if, else, for, switch, break, continue etc.). But both languages are different and have no relation with each other.

WHAT IS THE USE OF STOPPROPOGATION() METHOD ?

The stopPropogation method is used to stop the event from bubbling up the event chain. For example:- Any below nested divs with stopPropogation method use to prevent the default event propogation when clicked on the nested div.

WHAT IS THE USE OF SETINTERVAL() METHOD ?

When we want to call any function or evaluate an expression at continous specified intervals which is counted in milliseconds. For example:- Alert box we can able to popup continously after 2 seconds when the page loads.

WHAT IS ECMASCRIPT ?

It is a scripting language that forms the basis of JavaScript. We can say like it is the major revision to JavaScript. And every year they revised the older version of JavaScript so that JavaScript langauge can be more simplier and more advance.

Job Interview Questions Of JavaScript