JavaScript is a popular programming language used for developing web applications like Facebook, Google, etc. JS developers can now create server-side software because of NodeJS. Hence, many beginners want to learn JavaScript and pursue a career as a JavaScript developer.
However, it is always hard to crack interviews with multinational organizations because they require people with keen knowledge. So, if you also want to crack a JavaScript interview, you have come to the right place. Here we have covered the JavaScript interview questions, ranging from beginner to expert level. So let’s get started.
Top JavaScript Interview Questions
Here are the most commonly asked JavaScript questions, along with their appropriate answers.
Q.1 Explain the term “JavaScript”?
Unlike Java, JavaScript is a text-based scripting language. JavaScript allows you to implement more dynamic elements on web pages. As CSS and HTML provide style and structure to a web page, Javascript makes them more interactive. Javascript is used to develop both client and server-side applications.
Q.2 List some features of JavaScript.
These are some of the JavaScript features:
- Dynamic language
- Lightweight
- Case sensitive
- Event handling
- Interpreter based
- Supports functional programming
- Platform independent
- Client-side validations
Q.3 What are the different data types present in JavaScript?
There are mainly seven types of data types in JavaScript.
- Boolean
- Symbol
- Null
- String
- Undefined
- Biglnt
- Number
Q.4 What are the ways to create objects in JavaScript?
Through many ways, you can create objects in JavaScript:
| Object create method | var object = Object.create(null); |
| Object constructor | var object = new Object(); |
| Function constructor | function Person(name) {
this.name = name; this.age = 25; } var object = new Person(“Kartik”); |
| Object literal syntax | var object = {
name: “Kartik”, age: 25 }; |
| ES6 Class syntax | class Person {
constructor(name) { this.name = name; } } var object = new Person(“Kartik”); |
| Singleton pattern | var object = new (function () {
this.name = “Kartik”; })(); |
| Function constructor with prototype | function Person() {}
Person.prototype.name = “Kartik”; var object = new Person(); |
Q.5 What is the NaN property, and explain the use of the isNaN () function in JavaScript?
NaN stands for “Not-a-Number” in JavaScript. It’s not a valid number. The isNaN() function returns true when a value is NaN; otherwise, it returns false. It converts the value into a number before testing it.
isNaN(“JavaScript”) // Returns true
isNaN(123) // Returns false
Q.6 What are classes in JavaScript?
Classes are templates for creating objects; they were introduced in the ES6 version of Javascript. It encapsulates the data with code to operate on the data and provides a new way of declaring functions. JavaScript classes are built using prototypes. Prototypes are basically the base class for all the javascript objects. Classes in Javascript have semantics and syntax that are different from ES5 class-like semantics.
class human {} console.log(typeof human); // function
Q.7 What is meant by “this” in JavaScript?
The “this” keyword in JavaScript refers to an object that changes depending on how it is invoked (called or used). Also, we can say that “this” refers to the object executing the current part of the code.
Q.8 What’s the difference between JavaScript and Java?
The differences between JavaScript and Java are:
| Features | JavaScript | Java |
| Types & Trademarks | Oracle corporation owns the trademark for JavaScript, and JavaScript is a scripting language. | Java is a programming language whose trademark is also an Oracle corporation. |
| Browser Compatibility | Coding in JavaScript allows it to be compatible with multiple browsers. | Java is currently only supported by Internet Explorer. |
| Memory Usage | Consume less memory. | Consume more memory. |
| Development | It is used for back-end and front-end development. | It is mainly used for back-end development. |
| Multithreading | It doesn’t support multithreading. | It supports multithreading. |
Q.9 Explain Hoisting in JavaScript.
In JavaScript, Hoisting is a default behavior where all functions and declaration of variables are moved to the top, before the execution of the code. This means it is where functions and Java variables are declared and moved to the top of the scope. Here, the scope can be both global and local.
function doSomething(){
a = 36;
console.log(a);
var a;
}
Q.10 What is a prototype chain?
Prototypes in JavaScript create new types of objects based on existing chains. You can consider it as an inheritance if you are familiar with Object Oriented, class-based languages. The prototype on the constructor function is through Object.prototype, whereas the prototype on the object instance is available through Object.getPrototypeOf(object) or the **proto** property.
Q.11 Define closure.
In JavaScript, closure allows the user to access an outer function from an inner function. It combines a function enclosed (bundled together) with the lexical environment (the surrounding state).
var outerScope = function() {
var msg = “Hello World”;
var innerScope = function() {
console.log (msg);
}
return innerScope;
}
Q.12 What are the advantages and disadvantages of JavaScript?
The advantages and disadvantages of JavaScript are:
| Advantages of JavaScript | Disadvantages of JavaScript |
| Due to its client-side execution, JavaScript is very fast. | JavaScript’s DOM model (document object manipulation) is much slower than HTML. |
| Updated annually by ECMA. | Rendering a web page. |
| Easy to learn. | JavaScript code is visible to everyone, which is its major disadvantage. |
| It supports all modern browsers. | It only supports single inheritance. |
| Reduces the server load. |
Q.13 Why do we use the “debugger” in JavaScript?
The debugger keyword is used in JavaScript code to stop the execution of the code at the breaking point and calls for some debugging function. In cases where no debugging is required and no action is taken, the debugger runs the function.
Q.14 Would you consider JavaScript a statically typed or dynamically typed language?
JavaScript language is a dynamically typed language because it does not have only a few dynamic aspects, but everything is dynamic. Besides both existence and type variables being dynamic, the code in it is also dynamic. So, you can create new variables at runtime, and the type of these variables is determined at runtime.
Q.15 How are call (), apply (), and bind () different?
Call (): It is a predefined method that invokes a function (method) by specifying the owner object. This method also allows the object to use another object’s function (method).
Apply (): This method is similar to the call () method, but the call () method accepts the arguments separately. While this does not happen in the apply method, it accepts the arguments as arrays.
Bind (): The bind () method returns a new function where the “this” keyword is provided as a parameter whose value is bound to the owner object.
Q.16 What are BOM and DOM in JavaScript?
| BOM | DOM |
| BOM stands for browser object model, which supports window objects across all browsers. The window object also includes JavaScript objects, functions, and variables. The BOM refers to objects in the browser, such as the screen, history, navigator, and location. | The DOM in JavaScript refers to the document object model. It helps the user to access HTML and document elements. Browsers create DOMs for web pages when they load. |
Q.17 What do you understand about cookies, and how can you create a cookie?
The cookies are used to store data by a web browser. They can store a variety of information like your name, user data, and other personally identifiable information. Cookies are used to access the website. In addition to remembering information, these cookies record browsing activity.
You can create a cookie using JavaScript as follow:
document.cookie = “key1 = value1; key2 = value2; expires = date”;
Q.18 What’s the difference between let and var?
Both let and var are used for method and variable declarations in JavaScript. Due to this, there are a few differences between them. Let is scoped by a block, whereas functions scope var.
Q.19 What is the JavaScript method for reading and writing files?
You can read and write a file through JavaScript extensions, web pages, and Active X objects using JavaScript.
Q.20 How are the operators “==” and ” ===” different?
Both these operators are comparison operators. The only difference between these two is that “==” compares values while “===” compares both types and values.
Q.21 What are all the looping structures in JavaScript?
The looping structure in JavaScript is
- For
- While
- Do-while loops
Q.22 What are global variables and how are these variables declared?
Global variables are a special type of variable in JavaScript and are very easy to use. JavaScript also uses this variable throughout the code. You can declare a local or global variable using the var keyword.
Q.23 Explain the prompt box in JavaScript.
JavaScript uses a prompt box to allow users to enter values before entering a page. The prompt box provides a text box for the user to enter his input. It also provides a box and labels for entering numbers and text.
Q.24 How can JavaScript code be incorporated into HTML?
HTML files can include JavaScript code in three ways.
- Inline
- Internal
- External
Q.25 How to write HTML code and normal text code dynamically using JavaScript?
In JavaScript, you can dynamically write HTML and normal text code in the following way.
| HTML Code | Normal Text Code |
| Using the innerHTML property, you can write the HTML code. | Through the innerText property, you can write normal text code. |
| Example: document.getElementById(‘mylocation’).innerHTML=”<h2>This is heading using JavaScript</h2>”; | Example: document.getElementById(‘mylocation’).innerText=”This is text using JavaScript”; |
Q.26 List out different errors in JavaScript.
JavaScript errors can be classified into three main types.
- Syntax errors: Syntax errors occur when there are some mistakes in the program’s syntax body. This error is also known as a parsing error.
- Runtime errors: Runtime errors arise when the compiler interprets a program during its runtime.
- Logical error: Logical errors are the hardest because they are challenging to detect. Because in this error, the syntax is correct, but its logic is not.
Q.27 Why should you not use innerHTML in JavaScript?
The innerHTML content is always fresh, and hence it is also slow. There is no scope for verification, and it is very easy to destabilize the web page and insert fake code in the document, potentially damaging our website.
For all these reasons, you should not use innerHTML in JavaScript because it can be used for Cross-Site Scripting (XSS) to add client-side scripts and steal private user information stored in cookies.
Q.28 What is the MUL function in JavaScript?
A short form of the miniature multiplication function is the MUL function, in which the user calls a function that requires an argument as the first number. This function calls another function, and this second function requires a second argument; thus, the process continues.
Q.29 How to write a comment in JavaScript?
You can comment in JavaScript in two ways.
- You can write a single-line comment using // (double forward slash).
- You can write a multi-line comment with a slash asterisk as /* write a comment here */
Q.30 What is event bubbling?
In JavaScript, event propagation in the HTML DOM API is called event bubbling when an event occurs in an element inside another element. As the event bubbles up to the containing elements in the hierarchy, the method starts with the element that triggered the event.
Final Thoughts
If you are new to JavaScript, the interviewer will not ask you more complex questions, and their expectations from you will be practical. Just keep your basics strong. With all the above JavaScript interview questions, you will get in-depth knowledge of JavaScript to prepare for your JavaScript interview.
Hi! I am Shekhar, a professional web & mobile app developer with expertise in MEAN Stack, Next.js, React.js, and React Native. Being interested in working with different IT technologies, I always look forward to learning something new and challenging. Along with JavaScript, I also know several other programming languages, including Python and TypeScript.