Lesson 16 of 20By Aryan Gupta
Last updated on Mar 22, 20211145JavaScript is an open-source programming language. It is designed for creating web-centric applications. It is lightweight and interpreted, which makes it much faster than other languages. JavaScript is integrated with HTML, which makes it easier to implement JavaScript in web applications.
This article provides you with a comprehensive list of common JavaScript interview questions that often come up in interviews with great answers to them. It will also help you understand the fundamental concepts of JavaScript.
We have grouped the questions into the following categories:
Fig: JavaScript Logo
JavaScript is a popular web scripting language and is used for client-side and server-side development. The JavaScript code can be inserted into HTML pages that can be understood and executed by web browsers while also supporting object-oriented programming abilities.
JavaScript |
Java |
JavaScript is an object-oriented scripting language. |
Java is an object-oriented programming language. |
JavaScript applications are meant to run inside a web browser. |
Java applications are generally made for use in operating systems and virtual machines. |
JavaScript does not need compilation before running the application code. |
Java source code needs a compiler before it can be ready to run in realtime. |
These are the different types of data that JavaScript supports:
These are the features of JavaScript:
These are the advantages of JavaScript:
JavaScript adds interaction to otherwise static web pages and makes them react to users’ inputs.
There is no need for a web page to reload when running JavaScript. For example, form input validation.
JavaScript helps in making the UI of web applications look and feel much better.
JavaScript has countless frameworks and libraries that are extensively used for developing web applications and games of all kinds.
Since JavaScript is essentially an object-oriented scripting language, it supports and encourages the usage of objects while developing web applications.
const student = {
name: 'John',
age: 17
}
Here is a very simple way of creating arrays in JavaScript using the array literal:
var a = [];
var b = [‘a’, ‘b’, ‘c’, ‘d’, ‘e’];
Built-in Method |
Values |
Date() |
Returns the present date and time |
concat() |
Joins two strings and returns the new string |
push() |
Adds an item to an array |
pop() |
Removes and also returns the last element of an array |
round() |
Rounds of the value to the nearest integer and then returns it |
length() |
Returns the length of a string |
The scope of a variable implies where the variable has been declared or defined in a JavaScript program. There are two scopes of a variable:
Global variables, having global scope are available everywhere in a JavaScript code.
Local variables are accessible only within a function in which they are defined.
The ‘this’ keyword in JavaScript refers to the currently calling object. It is commonly used in constructors to assign values to object properties.
Following are the naming conventions for a variable in JavaScript:
In JavaScript, functions are objects and therefore, functions can take other functions as arguments and can also be returned by other functions.
Fig: Callback function
A callback is a JavaScript function that is passed to another function as an argument or a parameter. This function is to be executed whenever the function that it is passed to gets executed.
All modern web browsers like Chrome, Firefox, etc. have an inbuilt debugger that can be accessed anytime by pressing the relevant key, usually the F12 key. There are several features available to users in the debugging tools.
We can also debug a JavaScript code inside a code editor that we use to develop a JavaScript application—for example, Visual Studio Code, Atom, Sublime Text, etc.
Function declaration |
Function expression |
Declared as a separate statement within the main JavaScript code |
Created inside an expression or some other construct |
Can be called before the function is defined |
Created when the execution point reaches it; can be used only after that |
Offers better code readability and better code organization |
Used when there is a need for a conditional declaration of a function |
Example: function abc() { return 5; } |
Example: var a = function abc() { return 5; } |
There are primarily two ways of embedding JavaScript code:
Fig: Browser cookies
A cookie is generally a small data that is sent from a website and stored on the user’s machine by a web browser that was used to access the website. Cookies are used to remember information for later use and also to record the browsing activity on a website.
The simplest way of creating a cookie using JavaScript is as below:
document.cookie = "key1 = value1; key2 = value2; expires = date";
Reading a cookie using JavaScript is also very simple. We can use the document.cookie string that contains the cookies that we just created using that string.
The document.cookie string keeps a list of name-value pairs separated by semicolons, where ‘name’ is the name of the cookie, and ‘value’ is its value. We can also use the split() method to break the cookie value into keys and values.
To delete a cookie, we can just set an expiration date and time. Specifying the correct path of the cookie that we want to delete is a good practice since some browsers won’t allow the deletion of cookies unless there is a clear path that tells which cookie to delete from the user’s machine.
function delete_cookie(name) {
document.cookie = name + "=; Path=/; Expires=Thu, 01 Jan 1970 00:00:01 GMT;";
}
Both let and var are used for variable and method declarations in JavaScript. So there isn’t much of a difference between these two besides that while var keyword is scoped by function, the let keyword is scoped by a block.
Closures provide a better, and concise way of writing JavaScript code for the developers and programmers. Closures are created whenever a variable that is defined outside the current scope is accessed within the current scope.
function hello(name) {
var message = "hello " + name;
return function hello() {
console.log(message);
};
}
//generate closure
var helloWorld = hello("World");
//use closure
helloWorld();
Arrow functions are a short and concise way of writing functions in JavaScript. The general syntax of an arrow function is as below:
const helloWorld = () => {
console.log("hello world!");
};
Here are the ways an HTML element can be accessed in a JavaScript code:
There are three ways of defining a variable in JavaScript:
This is used to declare a variable and the value can be changed at a later time within the JavaScript code.
We can also use this to declare/define a variable but the value, as the name implies, is constant throughout the JavaScript program and cannot be modified at a later time.
This mostly implies that the values can be changed at a later time within the JavaScript code.
Imports and exports help in writing modular code for our JavaScript applications. With the help of imports and exports, we can split a JavaScript code into multiple files in a project. This greatly simplifies the application source code and encourages code readability.
calc.js
export const sqrt = Math.sqrt;
export function square(x) {
return x * x;
}
export function diag(x, y) {
return sqrt(square(x) + square(y));
}
This file exports two functions that calculate the squares and diagonal of the input respectively.
main.js
import { square, diag } from "calc";
console.log(square(4)); // 16
console.log(diag(4, 3)); // 5
Therefore, here we import those functions and pass input to those functions to calculate square and diagonal.
Document |
Window |
The document comes under the windows object and can also be considered as its property. |
Window in JavaScript is a global object that holds the structure like variables, functions, location, history, etc. |
JavaScript has a collection of many frameworks that aim towards fulfilling the different aspects of the web application development process. Some of the prominent frameworks are:
Undefined |
Undeclared |
Undefined means a variable has been declared but a value has not yet been assigned to that variable. |
Variables that are not declared or that do not exist in a program or application. |
Undefined |
Null |
Undefined means a variable has been declared but a value has not yet been assigned to that variable. |
Null is an assignment value that we can assign to any variable that is meant to contain no value. |
Session storage |
Local storage |
The data stored in session storage gets expired or deleted when a page session ends. |
Websites store some data in local machine to reduce loading time; this data does not get deleted at the end of a browsing session. |
There are a few ways in which we can empty an array in JavaScript:
var arr = [1, 2, 3, 4];
arr.length = 0;
var arr = [1, 2, 3, 4];
arr = [];
var arr = [1, 2, 3, 4];
while (arr.length > 0) {
arr.pop();
}
var arr = [1, 2, 3, 4];
arr.splice(0, arr.length);
Event Capturing |
Event Bubbling |
This process starts with capturing the event of the outermost element and then propagating it to the innermost element. |
This process starts with capturing the event of the innermost element and then propagating it to the outermost element. |
Strict mode in JavaScript introduces more stringent error-checking in a JavaScript code.
var a = 10;
if (function abc(){})
{
a += typeof abc;
}
console.log(a);
The output of this JavaScript code will be 10undefined. The if condition statement in the code evaluates using eval. Hence, eval(function abc(){}) will return function abc(){}.
Inside the if statement, executing typeof abc returns undefined because the if statement code executes at run time while the statement inside the if the condition is being evaluated.
<script type="text/javascript">
function addNode() {
var newP = document.createElement("p");
var textNode = document.createTextNode(" This is a new text node");
newP.appendChild(textNode); document.getElementById("firstP").appendChild(newP);
}
</script>
Call |
Apply |
In the call() method, arguments are provided individually along with a ‘this’ value. |
In the apply() method, arguments are provided in the form of an array along with a ‘this’ value. |
var Bar = Function Foo()
{
return 11;
};
typeof Foo();
The output would be a reference error since a function definition can only have a single reference variable as its name.
var Student = {
college: "abc",
};
var stud1 = Object.create(Student);
delete stud1.college;
console.log(stud1.company);
This is essentially a simple example of object-oriented programming. Therefore, the output will be ‘abc’ as we are accessing the property of the student object.
There are two ways in which we can remove duplicates from a JavaScript array:
To call the filter() method, three arguments are required. These are namely array, current element, and index of the current element.
An empty array is used for storing all the repeating elements.
As you prepare for your upcoming job interview, we hope that these JavaScript Interview Questions have provided more insight into what types of questions you are likely to be asked.
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Aryan is a tech enthusiast who likes to stay updated about trending technologies of today. He is passionate about all things technology, a keen researcher, and writes to inspire. Aside from technology, he is an active football player and a keen enthusiast of the game.
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